Only consider -1 as an error return value for open().

Only consider -1 as an error return value for open().
This is:
 - what Posix says (-1 on error, >=0 on success)
 - fixing a bug on winCE (<0 sometimes)

Patch by Martin Storsjö: martin martin st

Originally committed as revision 19513 to svn://svn.ffmpeg.org/ffmpeg/trunk