tools/target_dec_fuzzer: Adjust threshold for G2M

G2M allows large images from small input and also reallocates
multiple buffers on any resolution change.

Fixes: Timeout (22sec -> 5sec)
Fixes: 18022/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_G2M_fuzzer-5089192530411520

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