MPDStatus: Handle NaN for MixRampDelay.

Old versions of MPD, or maybe Voyage MPD alone, would report a float NaN with
MixRampDelay, which is non-standard with newer versions.
QueueFragment: Fix the playlist queue filter to inverse match.

Broken by ffde4dd.
CoverManager: Check for null retriever when traversing the coverRetriever list.

Check for null retriever when traversing the coverRetriever list, this can
cause an exception when the cover cache is disabled.