Verify UpdateTempDir isn't maliciously modified

Don't just delete the stored updates temporary directory on launch, but
validate the registry key to ensure it is in an expected location and
follows WinSparkle's naming convention. This is to prevent malicious
users from modifying this registry key and forcing the host app to
delete arbitrary directories (a user being able to delete the key would
be able to delete the directory themselves, so it's not a serious issue,
but it nevertheless is bad behavior and shouldn't be possible).