avcodec/mpegaudiodec_template: Make l3_unscale() work with e=0

Fixes undefined behavior
Fixes: 830/clusterfuzz-testcase-6253175327686656

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