search: Fix crash when clicking search playlist after startup.
Search playlists that are loaded with JuK on startup (i.e. because they were saved from a previous JuK invocation) cause a crash as soon as you click on the playlist after the recent merge request !18 (which refactors the search-handling code). The direct cause is that we were passing a pointer to a stack-allocated object (in the operator>> for SearchPlaylist), which caused a dangling pointer once the flow of execution left the stack frame. I did not catch this during the code review, but found a second instance of this in PlaylistCollection which I've also fixed.