ffmpeg: initialize got_output, this silences a compiler warning from icc

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

ffmpeg.c

@@ -1776,7 +1776,7 @@ static int transcode_subtitles(InputStream *ist, AVPacket *pkt, int *got_output)
 static int output_packet(InputStream *ist, const AVPacket *pkt)
 {
     int ret = 0, i;
-    int got_output;
+    int got_output = 0;
 
     AVPacket avpkt;
     if (!ist->saw_first_ts) {