wmavoice: truncate spillover_nbits if too large

This fixes triggering the av_assert0(ret <= tmp.size).

The problem was reintroduced by commit
7b27dd5c and originally fixed by
2a4700a4.

Reviewed-by: Ronald S. Bultje <rsbultje@gmail.com>
Signed-off-by: Andreas Cadhalpun <Andreas.Cadhalpun@googlemail.com>