Check for NULL commit buffer in cgit_parse_commit()

This can be NULL, so try not to segfault.

Signed-off-by: Lars Hjemli <hjemli@gmail.com>

parsing.c

@@ -200,6 +200,9 @@ struct commitinfo *cgit_parse_commit(struct commit *commit)
 	ret->subject = NULL;
 	ret->msg = NULL;
 
+	if (p == NULL)
+		return ret;
+
 	if (strncmp(p, "tree ", 5))
 		die("Bad commit: %s", sha1_to_hex(commit->object.sha1));
 	else