Fix misbehaviour in url_fseek() when seeking fails.

The return value of the seek function is -1 on error, not -EPIPE (the return value in url_seek() if no seek function pointer is set)
Patch by Ronald S. Bultje % rbultje A ronald P bitfreak P net %
Original thread:
date: Dec 31, 2006 9:25 PM
subject: [Ffmpeg-devel] Re: [PATCH] file length handling

Originally committed as revision 7766 to svn://svn.ffmpeg.org/ffmpeg/trunk