Do not use errno when pread() returns 0

Do not use errno when pread() returns 0

If we use pread() while at the end of the file, it will return 0, which is
not an error from the operating system point of view. In this case, errno
has not been set and must not be used.

Signed-off-by: Samuel Tardieu <sam@rfc1149.net>
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>