pgsql: Fix longstanding error in contrib/intarray's int[] & int[] opera

Fix longstanding error in contrib/intarray's int[] & int[] operator.

The array intersection code would give wrong results if the first entry of
the correct output array would be "1". (I think only this value could be
at risk, since the previous word would always be a lower-bound entry with
that fixed value.)

Problem spotted by Julien Rouhaud, initial patch by Guillaume Lelarge,
cosmetic improvements by me.

Branch
------
REL9_1_STABLE

Details
-------
http://git.postgresql.org/pg/commitdiff/86328cbc348a6c2614bd1fddbf21380a92417e04

Modified Files
--------------
contrib/intarray/_int_tool.c | 13 +++++++------
contrib/intarray/expected/_int.out | 6 ++++++
contrib/intarray/sql/_int.sql | 1 +
3 files changed, 14 insertions(+), 6 deletions(-)