Fix typo / editor bug or something like that.

While running code analysis tools this issue popped up. As it is written it is
clearly a NOP, but the reason for why it is written that way remains unknown.
I suspect that it was a textfragment introduced by some repeat-function of the
used editor (think '.' in VIM). The code was introduced by commit a932a7a801
and looked like this in the original form:

   if(!XQueryTree(FbTk::App::instance()->display(), window().window(),
                  &root_return, &parent_return,
                  &children_return, &nchildren_return))
       parent_return=parent_return;//return;

Look at the outcommented return statement. I decided to return from that
function instead of the NOP.

src/FbTk/Container.cc

@@ -129,7 +129,7 @@ bool Container::moveItemTo(Item item, int x, int y) {
     // get the root window
     if (!XQueryTree(display(), window(),
                     &root_return, &parent_return, &children_return, &nchildren_return))
-        parent_return = parent_return;
+        return false;
 
     if (children_return != NULL)
         XFree(children_return);