avcodec/g2meet: Check if adjusted pixel was on the stack

avcodec/g2meet: Check if adjusted pixel was on the stack

This basically checks if a pixel that was coded with prediction
and residual could have been stored using a previous case.
This avoids basically a string of 0 symbols stored in less than
50 bytes to hit a O(n²) codepath.

Fixes: Timeout (too slow to wait -> immediately)
Fixes: 8668/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_G2M_fuzzer-4895946310680576
Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>