ffmpeg: move a local variable definition later.

Fix a segfault when using the -map option without input files.
lavfi: fix filter format negotiation loop.

query_formats() returning EAGAIN is not considered a progress
in the format negotiation.

If the filter returns EAGAIN but did set some of its formats
lists, it could be considered a partial success and counted
as progress in the negotiation. Not counting it is not a
problem because it currently only happens in the first round,
where there will always be some progress on the other filters.
lavfi: add comments to explain the negotiation loop.
Merge remote-tracking branch 'cigaes/master'

* cigaes/master:
  lavfi: add comments to explain the negotiation loop.
  lavfi: fix filter format negotiation loop.
  ffmpeg: move a local variable definition later.

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