avcodec/speexdec: Consider mode in frame size check

No speex samples with non default frame sizes are known (to me)
the official speexenc seems to only generate the 3 default ones.
Thus it may be that the fuzzer samples where the first non default
values encountered by the decoder.
Possibly the "<" should be "!="

Fixes: out of array access
Fixes: 42821/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_SPEEX_fuzzer-5640695772217344

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

libavcodec/speexdec.c

@@ -1419,7 +1419,7 @@ static int parse_speex_extradata(AVCodecContext *avctx,
         return AVERROR_INVALIDDATA;
     s->bitrate = bytestream_get_le32(&buf);
     s->frame_size = bytestream_get_le32(&buf);
-    if (s->frame_size < NB_FRAME_SIZE)
+    if (s->frame_size < NB_FRAME_SIZE << s->mode)
         return AVERROR_INVALIDDATA;
     s->vbr = bytestream_get_le32(&buf);
     s->frames_per_packet = bytestream_get_le32(&buf);