gapi: Unblock from a waiting send on GAPI close

There was a race condition that would sometimes occur in that if we
stopped reading from the gapiChan (on shutdown) but then a new message
was available before we managed to close the GAPI, then we would wait
forever to finish the close because the channel never sent, and the
WaitGroup wouldn't let us exit.

This fixes this horrible, horrible race.

examples/lib/libmgmt3.go

@@ -154,7 +154,11 @@ func (obj *MyGAPI) Next() chan error {
 			select {
 			case <-ticker.C:
 				log.Printf("libmgmt: Generating new graph...")
-				ch <- nil // trigger a run
+				select {
+				case ch <- nil: // trigger a run
+				case <-obj.closeChan:
+					return
+				}
 			case <-obj.closeChan:
 				return
 			}