tls1_process_heartbeat: check for NULL after allocating buffer

Signed-off-by: Kurt Roeckx <kurt@roeckx.be>
Reviewed-by: Matt Caswell <matt@openssl.org>

ssl/ssl.h

@@ -2732,6 +2732,7 @@ void ERR_load_SSL_strings(void);
 #define SSL_F_TLS1_PREPARE_CLIENTHELLO_TLSEXT		 275
 #define SSL_F_TLS1_PREPARE_SERVERHELLO_TLSEXT		 276
 #define SSL_F_TLS1_PRF					 284
+#define SSL_F_TLS1_PROCESS_HEARTBEAT			 341
 #define SSL_F_TLS1_SETUP_KEY_BLOCK			 211
 #define SSL_F_TLS1_SET_SERVER_SIGALGS			 335