In av_close_input_stream(), flush the packet queue before to actually

close the stream.

This way the flushed packets can still reference the still unclosed
format context.

In particular this fixes a spurious error issued when closing the
video4linux2 buffer in mmap_release_buffer(), which tries to access
the file descriptor of an already closed file.

Originally committed as revision 26237 to svn://svn.ffmpeg.org/ffmpeg/trunk