Fix bug in brin_minmax_multi_union

When calling sort_expanded_ranges() we need to remember the return
value, because the function sorts and also deduplicates the ranges. So
the number of ranges may decrease. brin_minmax_multi_union failed to do
that, which resulted in crashes due to bogus ranges (equal minval/maxval
but not marked as compacted).

Reported-by: Jaime Casanova
Discussion: https://postgr.es/m/20210404052550.GA4376%40ahch-to

src/backend/access/brin/brin_minmax_multi.c

@@ -2794,7 +2794,7 @@ brin_minmax_multi_union(PG_FUNCTION_ARGS)
 											   BTLessStrategyNumber);
 
 	/* sort the expanded ranges */
-	sort_expanded_ranges(cmpFn, colloid, eranges, neranges);
+	neranges = sort_expanded_ranges(cmpFn, colloid, eranges, neranges);
 
 	/*
 	 * We've loaded two different lists of expanded ranges, so some of them