avcodec/escape124: Check depth against num_superblocks

Fixes: runtime error: left shift of 66184 by 15 places cannot be represented in type 'int'
Fixes: 1707/clusterfuzz-testcase-minimized-6502767008940032

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