swr: fix "may be used uninitialized in this function" warnings

The uninitialized use implicates 0 channels, which is unlikely
to reach this function but fixed anyway in this commit.

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
vc1dec: add assert to impossible *_valid combination.

This also fixes a warning about uninitialized variables

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