fix mpd/groovebasin id mapping memory leak
fix mpd/groovebasin id mapping memory leak

This leak occurred when an MPD client connected and changed the
playlist.

andrewrk/groovebasin,Qbuer/groovebasin,sammcj/groovebasin,CalebMorris/groovebasin,dwrensha/groovebasin,dwrensha/groovebasin,CalebMorris/groovebasin,dwrensha/groovebasin,gorlug/groovebasin,andrewrk/groovebasin,emersion/groovebasin,Qbuer/groovebasin,sammcj/groovebasin,neynah/groovebasin,melissanoelle/groovebasin,emersion/groovebasin,sammcj/groovebasin,neynah/groovebasin,gorlug/groovebasin,melissanoelle/groovebasin,CalebMorris/groovebasin,neynah/groovebasin,melissanoelle/groovebasin,gorlug/groovebasin,andrewrk/groovebasin,dwrensha/groovebasin,emersion/groovebasin,Qbuer/groovebasin