Add "Ref/deref DataEngines even if the metadata is invalid" (586ed85)

Just because the metadata is invalid doesn't mean the DataEngine
actually failed to load, we're still returning a valid object, and as
seen with the MPris DataEngine bug. It's important that we still release
it properly.

Add upstream patch as:
 Ref-deref-DataEngines-even-if-the-metadata-is-invalid.patch
This is related to KDE#374531

Gbp-Dch: Full