ui-shared: fix segfault when defbranch is NULL

Not sure if there's a better fix for this.  defbranch is
NULL here on my setup when a crawler hit an invalid URL,
causing strcmp to segfault.

Signed-off-by: Eric Wong <normalperson@yhbt.net>

ui-shared.c

@@ -253,7 +253,7 @@ static char *repolink(const char *title, const char *class, const char *page,
 		}
 		delim = "&";
 	}
-	if (head && strcmp(head, ctx.repo->defbranch)) {
+	if (head && ctx.repo->defbranch && strcmp(head, ctx.repo->defbranch)) {
 		html(delim);
 		html("h=");
 		html_url_arg(head);