Don't double panic in LinearSyncFence's destructor
Don't double panic in LinearSyncFence's destructor

TyOverby/glium,binsoftware/glium,bvssvni/glium,nicokoch/glium,cpjreynolds/glium,christopheraugustus/glium,tomaka/glium,caldwell/glium,carbidegames/glium,sdotz/glium,acolley/glium,glium/glium