comctl32/treeview: Return from TREEVIEW_LButtonDown when the treeview handle is invalid.
LButtonDown should return when the treeview handle is invalid (e.g. destroyed) after NM_CLICK to prevent further message processing.
Wine-Bug: https://bugs.winehq.org/show_bug.cgi?id=58211
-- v4: comctl32/treeview: Return from TREEVIEW_LButtonDown when the treeview handle is invalid. comctl32/tests: Add a test for treeview deletion during NM_CLICK in LBUTTONDOWN.