Add comments to NULL func ptrs in bio_method_st

This commit adds comments to bio_method_st definitions where the
function pointers are defined as NULL. Most of the structs have comments
but some where missing and not all consitent.

CLA: trivial
Reviewed-by: NPaul Dale <paul.dale@oracle.com>
Reviewed-by: NRichard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/4881)

crypto/bio/bss_acpt.c

@@ -61,11 +61,11 @@ static const BIO_METHOD methods_acceptp = {
     bread_conv,
     acpt_read,
     acpt_puts,
-    NULL,                       /* connect_gets, */
+    NULL,                       /* connect_gets,         */
     acpt_ctrl,
     acpt_new,
     acpt_free,
-    NULL,
+    NULL,                       /* connect_callback_ctrl */
 };
 
 const BIO_METHOD *BIO_s_accept(void)

crypto/bio/bss_conn.c

@@ -65,7 +65,7 @@ static const BIO_METHOD methods_connectp = {
     bread_conv,
     conn_read,
     conn_puts,
-    NULL,                       /* connect_gets, */
+    NULL,                       /* conn_gets, */
     conn_ctrl,
     conn_new,
     conn_free,

crypto/bio/bss_dgram.c

@@ -73,11 +73,11 @@ static const BIO_METHOD methods_dgramp = {
     bread_conv,
     dgram_read,
     dgram_puts,
-    NULL,                       /* dgram_gets, */
+    NULL,                       /* dgram_gets,         */
     dgram_ctrl,
     dgram_new,
     dgram_free,
-    NULL,
+    NULL,                       /* dgram_callback_ctrl */
 };
 
 # ifndef OPENSSL_NO_SCTP
@@ -91,11 +91,11 @@ static const BIO_METHOD methods_dgramp_sctp = {
     bread_conv,
     dgram_sctp_read,
     dgram_sctp_puts,
-    NULL,                       /* dgram_gets, */
+    NULL,                       /* dgram_gets,         */
     dgram_sctp_ctrl,
     dgram_sctp_new,
     dgram_sctp_free,
-    NULL,
+    NULL,                       /* dgram_callback_ctrl */
 };
 # endif
 

crypto/bio/bss_fd.c

@@ -71,7 +71,7 @@ static const BIO_METHOD methods_fdp = {
     fd_ctrl,
     fd_new,
     fd_free,
-    NULL,
+    NULL,                       /* fd_callback_ctrl */
 };
 
 const BIO_METHOD *BIO_s_fd(void)

crypto/bio/bss_file.c

@@ -56,7 +56,7 @@ static const BIO_METHOD methods_filep = {
     file_ctrl,
     file_new,
     file_free,
-    NULL,
+    NULL,                      /* file_callback_ctrl */
 };
 
 BIO *BIO_new_file(const char *filename, const char *mode)
@@ -408,7 +408,7 @@ static const BIO_METHOD methods_filep = {
     file_ctrl,
     file_new,
     file_free,
-    NULL,
+    NULL,                      /* file_callback_ctrl */
 };
 
 const BIO_METHOD *BIO_s_file(void)

crypto/bio/bss_log.c

@@ -90,14 +90,14 @@ static const BIO_METHOD methods_slg = {
     /* TODO: Convert to new style write function */
     bwrite_conv,
     slg_write,
-    NULL,
-    NULL,
+    NULL,                      /* slg_write_old,    */
+    NULL,                      /* slg_read,         */
     slg_puts,
     NULL,
     slg_ctrl,
     slg_new,
     slg_free,
-    NULL,
+    NULL,                      /* slg_callback_ctrl */
 };
 
 const BIO_METHOD *BIO_s_log(void)

crypto/bio/bss_mem.c

@@ -37,7 +37,7 @@ static const BIO_METHOD mem_method = {
     mem_ctrl,
     mem_new,
     mem_free,
-    NULL,
+    NULL,                      /* mem_callback_ctrl */
 };
 
 static const BIO_METHOD secmem_method = {
@@ -54,7 +54,7 @@ static const BIO_METHOD secmem_method = {
     mem_ctrl,
     secmem_new,
     mem_free,
-    NULL,
+    NULL,                      /* mem_callback_ctrl */
 };
 
 /* BIO memory stores buffer and read pointer  */

crypto/bio/bss_null.c

@@ -33,7 +33,7 @@ static const BIO_METHOD null_method = {
     null_ctrl,
     null_new,
     null_free,
-    NULL,
+    NULL,                     /* null_callback_ctrl */
 };
 
 const BIO_METHOD *BIO_s_null(void)

crypto/bio/bss_sock.c

@@ -44,11 +44,11 @@ static const BIO_METHOD methods_sockp = {
     bread_conv,
     sock_read,
     sock_puts,
-    NULL,                       /* sock_gets, */
+    NULL,                       /* sock_gets,         */
     sock_ctrl,
     sock_new,
     sock_free,
-    NULL,
+    NULL,                       /* sock_callback_ctrl */
 };
 
 const BIO_METHOD *BIO_s_socket(void)

crypto/comp/c_zlib.c

@@ -302,8 +302,8 @@ static const BIO_METHOD bio_meth_zlib = {
     /* TODO: Convert to new style read function */
     bread_conv,
     bio_zlib_read,
-    NULL,
-    NULL,
+    NULL,                      /* bio_zlib_puts, */
+    NULL,                      /* bio_zlib_gets, */
     bio_zlib_ctrl,
     bio_zlib_new,
     bio_zlib_free,

ssl/bio_ssl.c

@@ -37,11 +37,11 @@ static const BIO_METHOD methods_sslp = {
     BIO_TYPE_SSL,
     "ssl",
     ssl_write,
-    NULL,
+    NULL,                       /* ssl_write_old, */
     ssl_read,
-    NULL,
+    NULL,                       /* ssl_read_old,  */
     ssl_puts,
-    NULL,                       /* ssl_gets, */
+    NULL,                       /* ssl_gets,      */
     ssl_ctrl,
     ssl_new,
     ssl_free,