Make srcfile() return undef, if the file isn't there.

This has the advantage that it's now possible to check for the existence of a
sourcefile with that function.

IkiWiki.pm

@@ -743,7 +743,10 @@ sub srcfile_stat {
 }
 
 sub srcfile ($;$) {
-	return (srcfile_stat(@_))[0];
+	if (my @stat=srcfile_stat(@_)) {
+		return $stat[0];
+	}
+	return undef
 }
 
 sub add_underlay ($) {