kill uninitialised variable warning in ac3_probe()

Originally committed as revision 9521 to svn://svn.ffmpeg.org/ffmpeg/trunk

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;