app/headless: avoid race between ReleaseCurrent and MakeCurrent

Release context on MakeCurrent failure paths as well.

Fixes gio#144

Signed-off-by: Elias Naur <mail@eliasnaur.com>

app/headless/headless.go

@@ -139,8 +139,9 @@ func contextDo(ctx context, f func() error) error {
 			errCh <- err
 			return
 		}
-		defer ctx.ReleaseCurrent()
-		errCh <- f()
+		err := f()
+		ctx.ReleaseCurrent()
+		errCh <- err
 	}()
 	return <-errCh
 }