avformat/mvi: Use 64bit for testing dimensions

Fixes: signed integer overflow: 65535 * 65535 cannot be represented in type 'int'
Fixes: 26910/clusterfuzz-testcase-minimized-ffmpeg_dem_MVI_fuzzer-6649291124899840

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit 48fb752767)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavformat/mvi.c

@@ -94,7 +94,7 @@ static int read_header(AVFormatContext *s)
     vst->codecpar->codec_type = AVMEDIA_TYPE_VIDEO;
     vst->codecpar->codec_id   = AV_CODEC_ID_MOTIONPIXELS;
 
-    mvi->get_int = (vst->codecpar->width * vst->codecpar->height < (1 << 16)) ? avio_rl16 : avio_rl24;
+    mvi->get_int = (vst->codecpar->width * (int64_t)vst->codecpar->height < (1 << 16)) ? avio_rl16 : avio_rl24;
 
     mvi->audio_frame_size   = ((uint64_t)mvi->audio_data_size << MVI_FRAC_BITS) / frames_count;
     if (mvi->audio_frame_size <= 1 << MVI_FRAC_BITS - 1) {