comctl32/treeview: Protect TVM_GETITEM from invalid item pointers

[Nikolay Sivov]

Should fix regression http://bugs.winehq.org/show_bug.cgi?id=33001

Tests show that all versions after nt4 tolerate invalid pointers for 
items, so since we support TVM_GETITEM from another tree for a while now 
it bypassed called tree item validation.
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