comctl32/treeview: Fix a crash in TVS_SINGELEXPAND helper (Coverity)

[testbot at testbot.winehq.org]

Hi,

While running your changed tests on Windows, I think I found new failures.
Being a bot and all I'm not very good at pattern recognition, so I might be
wrong, but could you please double-check?
Full results can be found at
http://testbot.winehq.org/JobDetails.pl?Key=3980

Your paranoid android.


=== WNT4WSSP6 (32 bit treeview) ===
treeview.c:896: Test failed: got wrong state 60