advapi32: fix buffer overrun in tests/registry.c:wine_debugstr_wn(), try 2

[Dan Kegel]

OK, my last try was ugly and deserved to be dropped.
This one is a one-bit change, and easier to understand.

Fixes an off-by-one buffer overflow in wine_debugstr_wn()
in which the wchar after the end of the buffer was read.
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