imc: make issue non-fatal

The "faulty" samples actually sound fine when ignoring this issue.

For ticket #3886, more samples are decoded.

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
avcodec/imc: only check remaining bits in case bits will be read

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>