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_13_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/9e7432fafc9290792baa7fd4aff03387973b139b

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(-)