avcodec/truemotion1: Check that the input has enough space for a minimal index_stream

Fixes: Timeout (18sec -> 0.4sec)
Fixes: 17585/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_TRUEMOTION1_fuzzer-5117015135617024

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

libavcodec/truemotion1.c

@@ -444,6 +444,8 @@ static int truemotion1_decode_header(TrueMotion1Context *s)
     if (s->flags & FLAG_KEYFRAME) {
         /* no change bits specified for a keyframe; only index bytes */
         s->index_stream = s->mb_change_bits;
+        if (s->avctx->width * s->avctx->height / 2048 + header.header_size > s->size)
+            return AVERROR_INVALIDDATA;
     } else {
         /* one change bit per 4x4 block */
         s->index_stream = s->mb_change_bits +