InstallHinfSection broken

InstallHinfSection doesn't add REG_SZ entries. This happened in r17162.

SetupGetStringFieldW() (and possibly others too) now return failure and set an
error of ERROR_INSUFFICIENT_BUFFER when the passed in buffer and size are NULL
and 0 resp. I think this is not correct. MSDN says:

"If this function is called with a ReturnBuffer of NULL and a ReturnBufferSize
of zero, the function puts the buffer size needed to hold the specified data
into the variable pointed to by RequiredSize. If the function succeeds in this,
the return value is a nonzero value."

So buffer == NULL and size == 0 is a special case. Other code (e.g. the call in
lib/setupapi/install.c line 328) depends on this behaviour and is now broken too.