comctl32/tests: toolbar button text length and validity

comctl32/tests: toolbar button text length and validity

Windows API documentation suggests sending the TB_GETBUTTONTEXT message
with a NULL destination pointer to get the button text length. This is
currently not implemented properly in Wine.

Additionally, Windows ignores the iString field for separators, so even
if programs forget to initialize it and its bit pattern is an invalid
pointer, no crashes or failures can be seen. An uninitialized iString
field resembling an invalid pointer currently crashes Wine.

Older Windows versions (up to Windows 2003 inclusive) will not complain
if a non-separator button with an invalid pointer iString pointer is
added to a toolbar, although subsequently trying to access the button
text might cause a crash. In some versions of Vista the SendMessage will
fail but the program will not crash. In other versions of Vista, and in
Windows 2008, the program will crash on insertion.