pgsql: Fix integer-overflow problem in intarray's g_int_decompress().

Fix integer-overflow problem in intarray's g_int_decompress().

An array element equal to INT_MAX gave this code indigestion,
causing an infinite loop that surely ended in SIGSEGV.  We fixed
some nearby problems awhile ago (cf 757c5182f) but missed this.

Report and diagnosis by Alexander Lakhin (bug #18273); patch by me

Discussion: https://postgr.es/m/18273-9a832d1da122600c@postgresql.org

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/9034a2d5123498104ef1e515c0b56d70d98d0ce2

Modified Files
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contrib/intarray/_int_gist.c         | 10 ++++++----
contrib/intarray/data/test__int.data |  1 +
contrib/intarray/expected/_int.out   | 33 +++++++++++++++++----------------
contrib/intarray/sql/_int.sql        |  5 +++--
4 files changed, 27 insertions(+), 22 deletions(-)