lavfi/sendcmd: fix various "in in" rerepepetitions in messages
lavfi/sendcmd: drop now pointless sendcmd/asendcmd_init() wrappers
lavfi: add interleave filters
ffplay: check av_samples_get_buffer_size() return

Fixes CID1005310
Reviewed-by: Marton Balint <cus@passwd.hu>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>