Fix assert in BRIN build_distances

When brin_minmax_multi_union merges summaries, we may end up with just a
single range after merge_overlapping_ranges. The summaries may contain
just one range each, and they may overlap (or be exactly the same).

With a single range there's no distance to calculate, but we happen to
call build_distances anyway - which is fine, we don't calculate the
distance in this case, except that with asserts this failed due to a
check there are at least two ranges.

The assert is unnecessarily strict, so relax it a bit and bail out if
there's just a single range. The relaxed assert would be enough, but
this way we don't allocate unnecessary memory for distance.

Backpatch to 14, where minmax-multi opclasses were introduced.

Reported-by: Jaime Casanova
Backpatch-through: 14
Discussion: https://postgr.es/m/YzVA55qS0hgz8P3r@ahch-to

src/backend/access/brin/brin_minmax_multi.c

@@ -1335,7 +1335,11 @@ build_distances(FmgrInfo *distanceFn, Oid colloid,
 	int			ndistances;
 	DistanceValue *distances;
 
-	Assert(neranges >= 2);
+	Assert(neranges > 0);
+
+	/* If there's only a single range, there's no distance to calculate. */
+	if (neranges == 1)
+		return NULL;
 
 	ndistances = (neranges - 1);
 	distances = (DistanceValue *) palloc0(sizeof(DistanceValue) * ndistances);