avfilter/avfilter: fix null pointer dereference with queued ping filters

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
ffmpeg: print command replies starting on column 0

This fixes formating issues with multi-line replies

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
ffmpeg: improve built in docs for passing commands

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
ffmpeg: check for unsupported "queing of command on first filter supporting the specific commend"

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