Add a check for a failed malloc

Ensure we check for a NULL return from OPENSSL_malloc

Issue reported by Guido Vranken.

Reviewed-by: Richard Levitte <levitte@openssl.org>

ssl/d1_both.c

@@ -1459,6 +1459,8 @@ int dtls1_process_heartbeat(SSL *s)
          * plus 2 bytes payload length, plus payload, plus padding
          */
         buffer = OPENSSL_malloc(write_length);
+        if (buffer == NULL)
+            return -1;
         bp = buffer;
 
         /* Enter response type, length and copy payload */