pgsql: Don't reference out-of-bounds array elements in brin_minmax_mult

Don't reference out-of-bounds array elements in brin_minmax_multi.c

The primary fix here is to fix has_matching_range() so it does not
reference ranges->values[-1] when nranges == 0. Similar problems existed
in AssertCheckRanges() too. It does not look like any of these problems
could lead to a crash as the array in question is at the end of the Ranges
struct, and values[-1] is memory that belongs to other fields in the
struct. However, let's get rid of these rather unsafe coding practices.

In passing, I (David) adjusted some comments to try to make it more clear
what some of the fields are for in the Ranges struct. I had to study the
code to find out what nsorted was for as I couldn't tell from the
comments.

Author: Ranier Vilela
Discussion: https://postgr.es/m/CAEudQAqJQzPitufX-jR=YUbJafpCDAKUnwgdbX_MzSc93wuvdw@mail.gmail.com
Backpatch-through: 14, where multi-range brin was added.

Branch
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REL_14_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/c2aa5d01e3e33776f77e68a60ca5b60b58c5c674

Modified Files
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src/backend/access/brin/brin_minmax_multi.c | 172 ++++++++++++++--------------
1 file changed, 88 insertions(+), 84 deletions(-)