avcodec/amrwbdec: Fix 2 runtime errors: left shift of negative value -1

Fixes: 669/clusterfuzz-testcase-4847965409640448

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