avcodec/h263dec: fix "incompatible pointer type" warning

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
avcodec/xbmdec: fix pointer type warnings

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
avcodec/kgv1dec: add casts to silence "assignment from incompatible pointer type"

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
Initialize riff and wav size fields to -1 instead of 0.

WMP doess not play the output files if the fields are
set to 0 and not overwritten (using pipe output).

Fixes ticket #3346.