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_12_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/1c7443521f856dfb5f6731ff6fc0865a67a29f14

Modified Files
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contrib/intarray/_int_gist.c | 10 ++++++----
contrib/intarray/data/test__int.data | 1 +
contrib/intarray/expected/_int.out | 16 ++++++++--------
3 files changed, 15 insertions(+), 12 deletions(-)