It seems dual mono with a LFE channel is not forbidden

Fixes: 1570/clusterfuzz-testcase-minimized-6455337349545984

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/targets/ffmpegSigned-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit c55e6370)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>