avcodec/ilbcdec: Check startindex

Fixes: Out of array read
Fixes: 10789/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_ILBC_fuzzer-5153255445757952

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpegReviewed-by: Paul B Mahol <onemda@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/ilbcdec.c

@@ -1372,7 +1372,7 @@ static int ilbc_decode_frame(AVCodecContext *avctx, void *data,
 
     if (unpack_frame(s))
         mode = 0;
-    if (s->frame.start < 1)
+    if (s->frame.start < 1 || s->frame.start > 5)
         mode = 0;
 
     if (mode) {