#14 Wrong implementation of the SYM_TAB_PAIR.hash_code

[Eugene Melekhov]

The function hash_code used as result object_id which
in general can be negative. Negative value of the
hash_code breaks the XHASH_LIST logic.
Now it returns the object_id with high bit set to 0. This is
a temporary workaround and should be changed in the
future.