gitweb: skip logo in atom feed when there is none

With v1.5.0-rc0~169 (gitweb: Fix Atom feed <logo>: it is $logo,
not $logo_url, 2006-12-04), the logo URI to be written to Atom
feeds was corrected but the case of no logo forgotten.

Acked-by: Eric Wong <normalperson@yhbt.net>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Acked-by: Jakub Narebski <jnareb@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

gitweb/gitweb.perl

@@ -6870,7 +6870,7 @@ XML
 		if (defined $favicon) {
 			print "<icon>" . esc_url($favicon) . "</icon>\n";
 		}
-		if (defined $logo_url) {
+		if (defined $logo) {
 			# not twice as wide as tall: 72 x 27 pixels
 			print "<logo>" . esc_url($logo) . "</logo>\n";
 		}