Remove size_t cast in setting s->priv_data directly to the (integer) file

Remove size_t cast in setting s->priv_data directly to the (integer) file
descriptor returned by open(). This removes some dubious doublecasts such
as priv_data = (void *) (size_t) some_integer, and is always safe on systems
we care about because sizeof(int)<=sizeof(void*). See comments from Mans and
Michael in "[RFC] rtsp.c EOF support" thread.

Originally committed as revision 17768 to svn://svn.ffmpeg.org/ffmpeg/trunk