lavf: restore old behavior for custom AVIOContex with an AVFMT_NOFILE format.

av_open_input_stream used to allow this, even though it makes no sense.
Make it just print a warning instead of failing, thus restoring
compatibility.

Note that avformat_open_input() will still reject this combination.

Signed-off-by: Anton Khirnov <anton@khirnov.net>