ui-tag.c: do not segfault when id is missing from query-string

The purpose of the tag page is to print info about a specific tag,
but if no tag was specified on the query-string cgit used to segfault.

With this patch, cgit will fallback to the value of the 'h' parameter
instead (which is never NULL due to prepare_repo_cmd() in cgit.c).

It will now also verify that the specified tagname is in fact a valid
ref in the 'refs/tags/' namespace, i.e. specifying 'id=master' will
trigger a 'Bad tag reference' error.

Noticed-by: Eric Wong <normalperson@yhbt.net>
Signed-off-by: Lars Hjemli <hjemli@gmail.com>

ui-tag.c

@@ -37,7 +37,10 @@ void cgit_print_tag(char *revname)
 	struct tag *tag;
 	struct taginfo *info;
 
-	if (get_sha1(revname, sha1)) {
+	if (!revname)
+		revname = ctx.qry.head;
+
+	if (get_sha1(fmt("refs/tags/%s", revname), sha1)) {
 		cgit_print_error(fmt("Bad tag reference: %s", revname));
 		return;
 	}