core,w32: Fix missing sentinel in dir name builder.

* src/w32-util.c (_gpgme_get_gpgconf_path): Add NULL top strconcat.

Fortunately this is called early and the stack like cleared out so
that we have not seen wrong behaviour until now. We should really fix
all these annoying HANDLE/int cast warnings and alike so that real
bugs are not drowned by them.

• GnuPG-bug-id: T4369
• Signed-off-by: Werner Koch <wk@gnupg.org>