liblbx: Correct EOF handling in lbx_file_read.

liblbx: Correct EOF handling in lbx_file_read.

We should never see an EOF from the underlying I/O when reading archive
members, because we setup the read request to only return bytes that are
supposed to be there.

So if EOF happens, flag this as an error and do *not* set the eof flag
in the state structure.  Also fixup lbxtool to properly handle this.