avfilter/f_interleave: use the name 's' for the pointer to the private context

Signed-off-by: Paul B Mahol <onemda@gmail.com>
avformat/matroskadec: only set r_frame_rate if the value is within reasonable limits

Fixes Ticket2451

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