kill uninitialised variable warning in ac3_probe()

git-svn-id: file:///var/local/repositories/ffmpeg/trunk@9521 9553f0bf-9b14-0410-a0b8-cfaf0461ba5b

libavformat/raw.c

@@ -412,7 +412,7 @@ static int h261_probe(AVProbeData *p)
 
 static int ac3_probe(AVProbeData *p)
 {
-    int max_frames, first_frames, frames;
+    int max_frames, first_frames = 0, frames;
     uint8_t *buf, *buf2, *end;
     AC3HeaderInfo hdr;