demux_y4m: fix failure with nonseekable input

demux_y4m: fix failure with nonseekable input

Playback of nonseekable y4m streams was broken by a change merged from
svn, r30970 which added support for multiple yuv4mpeg files
concatenated together. The change included a stream_skip(stream, -1)
call which only works for seekable files. Work around this by
disabling the concatenated-file check for nonseekable streams. This
means concatenated files are only supported if the stream is seekable,
but this is an improvement compared to the svn commit which caused a
regression making _all_ files fail in the nonseekable case.

It would be reasonably easy to make the concatenation support work in
all cases, either by modifying demux_y4m or (probably better) adding a
general stream API to allow pushing back the read byte. However for
now I'm only fixing the regression with the above trivial change.