avcodec/mpeg4videodec: Fix overflow in virtual_ref computation

Fixes: runtime error: signed integer overflow: 262144 * -16120 cannot be represented in type 'int'
Fixes: 2292/clusterfuzz-testcase-minimized-6156080415506432

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