Resubmit user32: TrackPopupMenu(Ex) fixes

[Rein Klazes]

Hi,

Several rounds of commits have gone by without this patch.
I thought it was rather noncontroversial, there must
be something that I am overlooking. Can I get a hint please?

(cleaned up some white space errors, otherwise patch is unchanged. )

Fix another bug in the application of bug#9615. TrackMenuPopupEx is
called with a NULL menu handle, and the application crashes on the
returned NULL value in the following INITMENU message.

for the Changelog:
      Check for invalid menu handle passed to TrackPopupMenu and
TrackPopupMenuEx.
      Add conformance tests for the correct handling of those invalid 
handles.
      TrackPopupMenu should call TrackPopupMenuEx, not the other way.


Rein.