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>
(cherry picked from commit 87ddf9f1ef)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/4xm.c

@@ -525,6 +525,10 @@ static int decode_i_block(FourXContext *f, int16_t *block)
             break;
         if (code == 0xf0) {
             i += 16;
+            if (i >= 64) {
+                av_log(f->avctx, AV_LOG_ERROR, "run %d overflow\n", i);
+                return 0;
+            }
         } else {
             if (code & 0xf) {
                 level = get_xbits(&f->gb, code & 0xf);