Fix access zero memory if SSL_DEBUG is enabled

If compile OpenSSL with SSL_DEBUG macro, some test cases will cause the
process crashed in the debug code.
Reviewed-by: NTim Hudson <tjh@openssl.org>
Reviewed-by: NPaul Dale <paul.dale@oracle.com>
(Merged from https://github.com/openssl/openssl/pull/7707)

(cherry picked from commit 5a4481f0e0bbf836c7f41805e84617784cb6b1db)

ssl/statem/statem_clnt.c

@@ -2353,7 +2353,8 @@ MSG_PROCESS_RETURN tls_process_key_exchange(SSL *s, PACKET *pkt)
         }
 #ifdef SSL_DEBUG
         if (SSL_USE_SIGALGS(s))
-            fprintf(stderr, "USING TLSv1.2 HASH %s\n", EVP_MD_name(md));
+            fprintf(stderr, "USING TLSv1.2 HASH %s\n",
+                    md == NULL ? "n/a" : EVP_MD_name(md));
 #endif
 
         if (!PACKET_get_length_prefixed_2(pkt, &signature)

ssl/statem/statem_lib.c

@@ -396,7 +396,8 @@ MSG_PROCESS_RETURN tls_process_cert_verify(SSL *s, PACKET *pkt)
 
 #ifdef SSL_DEBUG
     if (SSL_USE_SIGALGS(s))
-        fprintf(stderr, "USING TLSv1.2 HASH %s\n", EVP_MD_name(md));
+        fprintf(stderr, "USING TLSv1.2 HASH %s\n",
+                md == NULL ? "n/a" : EVP_MD_name(md));
 #endif
 
     /* Check for broken implementations of GOST ciphersuites */
@@ -439,7 +440,8 @@ MSG_PROCESS_RETURN tls_process_cert_verify(SSL *s, PACKET *pkt)
     }
 
 #ifdef SSL_DEBUG
-    fprintf(stderr, "Using client verify alg %s\n", EVP_MD_name(md));
+    fprintf(stderr, "Using client verify alg %s\n",
+            md == NULL ? "n/a" : EVP_MD_name(md));
 #endif
     if (EVP_DigestVerifyInit(mctx, &pctx, md, NULL, pkey) <= 0) {
         SSLfatal(s, SSL_AD_INTERNAL_ERROR, SSL_F_TLS_PROCESS_CERT_VERIFY,