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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REL_14_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/90e8b86fc52a4be21aec16abfb5dccfb819cc53e

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(-)