Don't ignore must_exist in open_registry().

Don't ignore must_exist in open_registry().

A typo meant that we were passing true to the overloaded open_registry()
which also takes an HKEY as a parameter.

As a result we would end up spamming ERROR_FILE_NOT_FOUND when updating
a service via the GUI if there were no hooks to update, ironically in a
section of code whose purpose was to check that it didn't need to do or
report anything.

Thanks Igor Zenkov and James Gleason.