avcodec/mobiclip: Check quantizer before table setup

avcodec/mobiclip: Check quantizer before table setup

Fixes: index -1 out of bounds for type 'const uint8_t [6][16]'
Fixes: out of array read
Fixes: shift exponent -21 is negative
Fixes: 25422/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_MOBICLIP_fuzzer-5748258226569216
Fixes: shift exponent 8039082 is too large for 32-bit type 'int'
Fixes: 25430/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_MOBICLIP_fuzzer-5698567770210304
Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Reviewed-by: Paul B Mahol <onemda@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>