D7750: fix crash when more than one instances of ExtractorCollection are destructed

[Matthieu Gallien]

mgallien marked an inline comment as done.

REPOSITORY
  R286 KFileMetaData

REVISION DETAIL
  https://phabricator.kde.org/D7750

To: mgallien, #frameworks, dfaure
Cc: dfaure, anthonyfieroni
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