Certain parameters, like calculated framerate, are unavailable when connecting
the output of a filtergraph to the input of another.
This fixes command lines like
ffmpeg -lavfi "testsrc=rate=1:duration=1[out0]" -filter_complex "[out0]null[out1]" -map [out1] -y out.png
Signed-off-by: James Almer <jamrial@gmail.com>