Fix segmentation fault in hc()

The ctx.qry.page variable might be unset at this point, e.g. when an
invalid command is passed and cgit_print_pageheader() is called to show
an error message.

Signed-off-by: Lukas Fleischer <lfleischer@lfos.de>

ui-shared.c

@@ -889,6 +889,9 @@ void cgit_add_hidden_formfields(int incl_head, int incl_search,
 
 static const char *hc(const char *page)
 {
+	if (!ctx.qry.page)
+		return NULL;
+
 	return strcmp(ctx.qry.page, page) ? NULL : "active";
 }