avcodec/flicvideo: Fix runtime error: signed integer overflow: 4864 * 459296 cannot be represented in type 'int'

Fixes: 2174/clusterfuzz-testcase-minimized-5739234533048320

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