Fix the BIO callback return code handling

The BIO callback handling incorrectly wrote over the return code passed
to the callback, meaning that an incorrect result was (eventually) returned
to the caller.

Fixes #7343
Reviewed-by: NTim Hudson <tjh@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/7344)

(cherry picked from commit d97ce8d9a0619c1d9d1222dc1b44dbebb58dd966)

crypto/bio/bio_lib.c

@@ -52,7 +52,7 @@ static long bio_call_callback(BIO *b, int oper, const char *argp, size_t len,
         argi = (int)len;
     }
 
-    if (inret && (oper & BIO_CB_RETURN) && bareoper != BIO_CB_CTRL) {
+    if (inret > 0 && (oper & BIO_CB_RETURN) && bareoper != BIO_CB_CTRL) {
         if (*processed > INT_MAX)
             return -1;
         inret = *processed;
@@ -60,7 +60,7 @@ static long bio_call_callback(BIO *b, int oper, const char *argp, size_t len,
 
     ret = b->callback(b, oper, argp, argi, argl, inret);
 
-    if (ret >= 0 && (oper & BIO_CB_RETURN) && bareoper != BIO_CB_CTRL) {
+    if (ret > 0 && (oper & BIO_CB_RETURN) && bareoper != BIO_CB_CTRL) {
         *processed = (size_t)ret;
         ret = 1;
     }