avfilter/af_asetnsamples: fix pts handling when timebase and 1/samplerate differ

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
swr: remove unnecessary assignment.

I don't see dst_incr/dst_incr_frac ever being changed from their
initial value (which is the inverse of this operation), so it seems
to me that this is a no-op.

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