Allow double close of codecs (#364)

Video/AudioTrack.NewRTPReader() internally closes encoder on error.
It caused double free if user closes reader after error.

pkg/codec/vaapi/vaapi_test.go

@@ -51,6 +51,20 @@ func TestEncoder(t *testing.T) {
 					),
 				)
 			})
+			t.Run("CloseTwice", func(t *testing.T) {
+				p, err := factory()
+				if err != nil {
+					t.Fatal(err)
+				}
+				codectest.VideoEncoderCloseTwiceTest(t, p, prop.Media{
+					Video: prop.Video{
+						Width:       640,
+						Height:      480,
+						FrameRate:   30,
+						FrameFormat: frame.FormatI420,
+					},
+				})
+			})
 		})
 	}
 }