gitweb: fix 'Use of uninitialized value' error in href()

Equality between file_parent and file_name was being checked without a
preliminary check for existence of the parameters.

Fix by wrapping the equality check in appropriate if (defined ...),
rearranging the lines to prevent excessive length.

Signed-off-by: Giuseppe Bilotta <giuseppe.bilotta@gmail.com>
Acked-by: Jakub Narebski <jnareb@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

gitweb/gitweb.perl

@@ -940,10 +940,13 @@ sub href {
 			if (defined $params{'hash_parent_base'}) {
 				$href .= esc_url($params{'hash_parent_base'});
 				# skip the file_parent if it's the same as the file_name
-				delete $params{'file_parent'} if $params{'file_parent'} eq $params{'file_name'};
-				if (defined $params{'file_parent'} && $params{'file_parent'} !~ /\.\./) {
-					$href .= ":/".esc_url($params{'file_parent'});
-					delete $params{'file_parent'};
+				if (defined $params{'file_parent'}) {
+					if (defined $params{'file_name'} && $params{'file_parent'} eq $params{'file_name'}) {
+						delete $params{'file_parent'};
+					} elsif ($params{'file_parent'} !~ /\.\./) {
+						$href .= ":/".esc_url($params{'file_parent'});
+						delete $params{'file_parent'};
+					}
 				}
 				$href .= "..";
 				delete $params{'hash_parent'};