Make SSL_CIPHER_get_version return a const char *

Reviewed-by: NViktor Dukhovni <viktor@openssl.org>

MR: #1595

doc/ssl/ssl.pod

@@ -206,7 +206,7 @@ Return the internal name of I<cipher> as a string. These are the various
 strings defined by the I<SSL3_TXT_xxx> and I<TLS1_TXT_xxx>
 definitions in the header files.
 
-=item char *B<SSL_CIPHER_get_version>(SSL_CIPHER *cipher);
+=item const char *B<SSL_CIPHER_get_version>(SSL_CIPHER *cipher);
 
 Returns a string like "C<SSLv3>" or "C<TLSv1.2>" which indicates the
 SSL/TLS protocol version to which I<cipher> belongs (i.e. where it was defined

include/openssl/ssl.h

@@ -1397,7 +1397,7 @@ void SSL_CTX_flush_sessions(SSL_CTX *ctx, long tm);
 
 __owur const SSL_CIPHER *SSL_get_current_cipher(const SSL *s);
 __owur int SSL_CIPHER_get_bits(const SSL_CIPHER *c, int *alg_bits);
-__owur char *SSL_CIPHER_get_version(const SSL_CIPHER *c);
+__owur const char *SSL_CIPHER_get_version(const SSL_CIPHER *c);
 __owur const char *SSL_CIPHER_get_name(const SSL_CIPHER *c);
 __owur uint32_t SSL_CIPHER_get_id(const SSL_CIPHER *c);
 __owur int SSL_CIPHER_get_kx_nid(const SSL_CIPHER *c);

ssl/ssl_ciph.c

@@ -1807,7 +1807,7 @@ char *SSL_CIPHER_description(const SSL_CIPHER *cipher, char *buf, int len)
     return (buf);
 }
 
-char *SSL_CIPHER_get_version(const SSL_CIPHER *c)
+const char *SSL_CIPHER_get_version(const SSL_CIPHER *c)
 {
     uint32_t alg_ssl;