advapi32[7/8]: fix GetServiceDisplayName for cchBuffer == 0

[Mikolaj Zalewski]

With the current code if one passes 0 as the buffer size it won't be
updated to contain the correct buffer size. The affects e.g. our
GetServieKeyName test. This patch fixes it and simplifies the logic -
the new code passes all the tests, seems correct but is simpler.
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