avoid uninitialised value

IkiWiki/Plugin/editdiff.pm

@@ -46,10 +46,10 @@ sub diff ($$) { #{{{
 sub formbuilder_setup { #{{{
 	my %params=@_;
 	my $form=$params{form};
+
+	return if defined ! $form->field("do") || $form->field("do") ne "edit";
+
 	my $page=$form->field("page");
-
-	return if $form->field("do") ne "edit";
-
 	$page = IkiWiki::possibly_foolish_untaint($page);
 	return unless exists $pagesources{$page};