gitweb: fix another use of undefined value

Pasky and Jakub competed fixing these and in the confusion this ended up
not being covered.

Signed-off-by: Junio C Hamano <junkio@cox.net>

gitweb/gitweb.perl

@@ -1114,7 +1114,9 @@ sub git_get_project_description {
 	open my $fd, "$projectroot/$path/description" or return undef;
 	my $descr = <$fd>;
 	close $fd;
-	chomp $descr;
+	if (defined $descr) {
+		chomp $descr;
+	}
 	return $descr;
 }