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: /messages/by-id/18273-9a832d1da122600c@postgresql.org

Branch
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REL_15_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/940ab02b53eb3a3babc9dd4dea261f5a6d8aa334

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(-)