advapi32[3/3]: fix GetServiceDisplayNameA for cchBuffer == 0

[Mikolaj Zalewski]

This patch is not connected to the previous patches but I thought to
send it. We have a bug in GetServiceDisplayNameA for cchBuffer == 0
that affect e.g. our GetKeyDisplayName test. This patch fixes it and
make the GetServiceDisplayNameA a bit simpler while passing all the
tests.
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