pgsql: Add overflow checks to int4 and int8 versions of generate_series

Add overflow checks to int4 and int8 versions of generate_series().

The previous code went into an infinite loop after overflow.  In fact,
an overflow is not really an error; it just means that the current
value is the last one we need to return.  So, just arrange to stop
immediately when overflow is detected.

Back-patch all the way.

Branch
------
REL8_2_STABLE

Details
-------
http://git.postgresql.org/pg/commitdiff/199d449c076a8564a9e8b113818f71c43f59d4cd

Modified Files
--------------
src/backend/utils/adt/int.c  |    4 ++++
src/backend/utils/adt/int8.c |    4 ++++
2 files changed, 8 insertions(+), 0 deletions(-)