widgets/msgview: Reap the filter command

The filter command shells out and returns almost immediately. Call
Wait() so the filter process gets reaped. Prior to this patch, aerc
creates a zombie process for every email that is viewed.

Signed-off-by: Kevin Kuehler <keur@xcf.berkeley.edu>

widgets/msgviewer.go

@@ -537,6 +537,7 @@ func (pv *PartViewer) attemptCopy() {
 			go func() {
 				<-ch
 				<-ch
+				pv.filter.Wait()
 				pv.pagerin.Close()
 			}()
 		}