pgsql: Tweak the overflow checks in integer division functions to

Log Message:
-----------
Tweak the overflow checks in integer division functions to complain if the
machine produces zero (rather than the more usual minimum-possible-integer)
for the only possible overflow case. This has been seen to occur for at least
some word widths on some hardware, and it's cheap enough to check for
everywhere. Per Peter's analysis of buildfarm reports.

This could be back-patched, but in the absence of any gripes from the field
I doubt it's worth the trouble.

Modified Files:
--------------
pgsql/src/backend/utils/adt:
int.c (r1.82 -> r1.83)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/int.c?r1=1.82&r2=1.83)
int8.c (r1.70 -> r1.71)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/int8.c?r1=1.70&r2=1.71)