avcodec/4xm: Check index in decode_i_block() also in the path where its not used.

avcodec/4xm: Check index in decode_i_block() also in the path where its not used.

Fixes: Infinite loop
Fixes: signed integer overflow: 2147483644 + 16 cannot be represented in type 'int'
Fixes: 16169/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_FOURXM_fuzzer-5662570416963584
Fixes: 16782/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_FOURXM_fuzzer-5743163859271680
Fixes: 17641/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_FOURXM_fuzzer-5711603562971136
Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Reviewed-by: Paul B Mahol <onemda@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>