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_16_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/cf6f802bf538ab1c9c0ef60a6e9d6fafdfb06b98

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(-)