img2: return AVERROR(EINVAL) in case of invalid filename pattern

In write_packet(), return AVERROR(EINVAL) rather than AVERROR(EIO) if
the provided output filename pattern is not valid. Indeed this has to
be considered a configuration error rather than an I/O error.

Allow a less confusing error report.

Signed-off-by: Stefano Sabatini <stefano.sabatini-lala@poste.it>
