avoid nil pointer dereference panic when freeing unavailable stream

apiGo/videoparser/thumbnail/Thumbnailparser_shared_ffmpeg.go

@@ -177,9 +177,11 @@ func decodePic(srcFileName string, decodeExtension string, time uint64) (pic *[]
 	}
 
 	for i := 0; i < inputCtx.StreamsCnt(); i++ {
-		st, _ := inputCtx.GetStream(i)
+		st, err := inputCtx.GetStream(i)
+		if err == nil && st != nil {
 			st.CodecCtx().Free()
 			st.Free()
 		}
+	}
 	return
 }