Set GITHUB_TAG unconditionally if found and avoid to use undef variables.

Pointed out by coypu via:

 <http://mail-index.netbsd.org/pkgsrc-users/2016/07/26/msg023569.html>

TODO: strictly this is not needed when GITHUB_TAG == PKGVERSION_NOREV
TODO: So maybe a better idea is to instruct url2pkg to understand the
TODO: PKGVERSION_NOREV in order to avoid that.

Index: Makefile
===================================================================
RCS file: /cvsroot/pkgsrc/pkgtools/url2pkg/Makefile,v
retrieving revision 1.85
diff -u -p -r1.85 Makefile
--- Makefile	10 Jul 2016 13:31:54 -0000	1.85
+++ Makefile	26 Jul 2016 15:10:10 -0000
@@ -1,7 +1,6 @@
 # $NetBSD: Makefile,v 1.85 2016/07/10 13:31:54 rillig Exp $
 
-PKGNAME=	url2pkg-2.27
-PKGREVISION=	1
+PKGNAME=	url2pkg-2.28
 CATEGORIES=	pkgtools
 
 MAINTAINER=	pkgsrc-users@NetBSD.org
Index: files/url2pkg.pl
===================================================================
RCS file: /cvsroot/pkgsrc/pkgtools/url2pkg/files/url2pkg.pl,v
retrieving revision 1.33
diff -u -p -r1.33 url2pkg.pl
--- files/url2pkg.pl	26 Jun 2016 19:03:33 -0000	1.33
+++ files/url2pkg.pl	26 Jul 2016 15:10:10 -0000
@@ -304,6 +304,7 @@ sub generate_initial_package($) {
 	}
 
 	if (!$found) {
+		$gh_project = $gh_release = $dist_subdir = "";
 		if ($url =~ qr"^https?://github\.com/") {
 			if ($url =~ qr"^https?://github\.com/(.*)/(.*)/archive/(.*)(\.tar\.gz|\.zip)$") {
 				$master_sites = "\${MASTER_SITE_GITHUB:=$1/}";
@@ -333,8 +334,6 @@ sub generate_initial_package($) {
 			} else {
 				print("$0: ERROR: Invalid GitHub URL: ${url}, handling as normal URL\n");
 			}
-		} else {
-			$gh_project = ""; $gh_release = ""; $dist_subdir = "";
 		}
 	}
 
@@ -374,6 +373,9 @@ sub generate_initial_package($) {
 		(($gh_release ne "")
 		? ["GITHUB_RELEASE", $gh_release]
 		: ()),
+		(($gh_tag ne "")
+		? ["GITHUB_TAG", $gh_tag]
+		: ()),
 		(($dist_sufx ne ".tar.gz" && $dist_sufx ne ".gem")
 		? ["EXTRACT_SUFX", $dist_sufx]
 		: ()),