Fix confusion between size_t and ssize_t

Fix confusion between size_t and ssize_t

readfile returns a value of type ssize_t (signed) and returns -1 if an
error occurs.  In auth_readdb_internal, however, we were assigning the
return value of readfile to a variable of type size_t (unsigned), but
then testing this unsigned value to see if it was < 0, a
contradiction.  We would thus simultaneously fail to report the error
in readfile and would end up with a corrupted length value.