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

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.