OLEAUT32: fix concatenation of VT_BSTR and non-VT_BSTR in VarCat

[Alex Villaci­s Lasso]

This bug arose while testing why text was being corrupted in the 
VisualBasic application I was using for testing the Unicode 
implementation for builtin ODBC. I found that in the VarCat function in 
variant.c, a "bstrvar" VARIANT was declared, but its V_VT was not being 
initialized. In addition, neither VariantChangeTypeEx or VariantCopy 
took the trouble of setting the correct value for the destination 
VARIANT, so this temporary remained with a bogus V_VT which was 
propagated up to the application. From the VARIANT tests in the tests/ 
directory, it seems that setting the destination V_VT is not the 
responsibility of VariantCopy, since all the test set it explicitly. I 
thought that VariantChangeTypeEx would be an appropriate place, but this 
crashed another dialog on the same VisualBasic application. So the 
assignment ended up in VarCat.

Changelog:
* VarCat now explicitly sets V_VT of temporary before calling 
VariantChangeTypeEx

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