avcodec/v4l2_buffers: Fix infinite loop

avcodec/v4l2_buffers: Fix infinite loop

This part of the code counts the number of planes returned by the v4l2
device for each queried capture/output buffer.
When testing the GPU h264 encoder on Nvidia's Jetson Nano, this caused an
infinite loop because avbuf->buf.length included some empty buffers (i.e.
where avbuf->buf.m.planes[i].length = 0), meaning that the counter was
never incremented and break was never reached.
This is fixed in the commit by using a well defined iteration range.

Signed-off-by: Aman Gupta <aman@tmm1.net>