dtls1_process_heartbeat: check for NULL after allocating buffer

Signed-off-by: NKurt Roeckx <kurt@roeckx.be>
Reviewed-by: NRichard Levitte <levitte@openssl.org>

ssl/d1_both.c

@@ -1392,6 +1392,8 @@ dtls1_process_heartbeat(SSL *s)
 		 * payload, plus padding
 		 */
 		buffer = OPENSSL_malloc(write_length);
+		if (buffer == NULL)
+			return -1;
 		bp = buffer;
 
 		/* Enter response type, length and copy payload */