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
------
REL8_3_STABLE

Details
-------
http://git.postgresql.org/pg/commitdiff/b0e1a4bd5e1d2057d0ddab9b93f2cccb538989dc

Modified Files
--------------
contrib/intarray/_int_tool.c | 15 +++++++++------
contrib/intarray/expected/_int.out | 6 ++++++
contrib/intarray/sql/_int.sql | 1 +
3 files changed, 16 insertions(+), 6 deletions(-)