pgsql: Guard against division by zero in test_int128 module.

Guard against division by zero in test_int128 module.

When testing 128/32-bit division in the test_int128 test module, make
sure that we don't attempt to divide by zero.

While at it, use pg_prng_int64() and pg_prng_int32() to generate the
random numbers required for the tests, rather than pg_prng_uint64().
This eliminates the need for any implicit or explicit type casts.

Author: Dean Rasheed <dean(dot)a(dot)rasheed(at)gmail(dot)com>
Reported-by: Andres Freund <andres(at)anarazel(dot)de>
Discussion: https://postgr.es/m/da4wqngd6gwz5s4yf5y5f75xj7gpja62l4dbp6w4j3vs7fcjue@upvolcu4e6o2

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/7e2af1fb1118a547bd831fca3afbc3ff80ebf089

Modified Files
--------------
src/test/modules/test_int128/test_int128.c | 14 +++++++++-----
1 file changed, 9 insertions(+), 5 deletions(-)