add. test codec ctx

cgo/ffmpeg/audio.go

@@ -585,6 +585,11 @@ func (self *AudioDecoder) Decode(pkt []byte) (gotframe bool, frame av.AudioFrame
 
 	cgotframe := C.int(0)
 
+	if ff.codecCtx == nil {
+		err = fmt.Errorf("ffmpeg: Decode ff.codecCtx NULL")
+		return 
+	}
+
 	cerr := C.decode(ff.codecCtx, ff.frame, (*C.uchar)(unsafe.Pointer(&pkt[0])), C.int(len(pkt)), &cgotframe)
 
 	if cerr < C.int(0) {