Fix longstanding error in contrib/intarray's int[] & int[] operator.

The array intersection code would give wrong results if the first entry of
the correct output array would be "1".  (I think only this value could be
at risk, since the previous word would always be a lower-bound entry with
that fixed value.)

Problem spotted by Julien Rouhaud, initial patch by Guillaume Lelarge,
cosmetic improvements by me.

contrib/intarray/_int_tool.c

@@ -140,7 +140,8 @@ inner_int_inter(ArrayType *a, ArrayType *b)
 			   *db,
 			   *dr;
 	int			i,
-				j;
+				j,
+				k;
 
 	if (ARRISEMPTY(a) || ARRISEMPTY(b))
 		return new_intArrayType(0);
@@ -152,15 +153,15 @@ inner_int_inter(ArrayType *a, ArrayType *b)
 	r = new_intArrayType(Min(na, nb));
 	dr = ARRPTR(r);
 
-	i = j = 0;
+	i = j = k = 0;
 	while (i < na && j < nb)
 	{
 		if (da[i] < db[j])
 			i++;
 		else if (da[i] == db[j])
 		{
-			if (i + j == 0 || (i + j > 0 && *(dr - 1) != db[j]))
-				*dr++ = db[j];
+			if (k == 0 || dr[k - 1] != db[j])
+				dr[k++] = db[j];
 			i++;
 			j++;
 		}
@@ -168,13 +169,13 @@ inner_int_inter(ArrayType *a, ArrayType *b)
 			j++;
 	}
 
-	if ((dr - ARRPTR(r)) == 0)
+	if (k == 0)
 	{
 		pfree(r);
 		return new_intArrayType(0);
 	}
 	else
-		return resize_intArrayType(r, dr - ARRPTR(r));
+		return resize_intArrayType(r, k);
 }
 
 void

contrib/intarray/expected/_int.out

@@ -137,6 +137,12 @@ SELECT '{123,623,445}'::int[] & '{1623,623}';
  {623}
 (1 row)
 
+SELECT '{-1,3,1}'::int[] & '{1,2}';
+ ?column? 
+----------
+ {1}
+(1 row)
+
 --test query_int
 SELECT '1'::query_int;
  query_int 

contrib/intarray/sql/_int.sql

@@ -24,6 +24,7 @@ SELECT '{123,623,445}'::int[] | 623;
 SELECT '{123,623,445}'::int[] | 1623;
 SELECT '{123,623,445}'::int[] | '{1623,623}';
 SELECT '{123,623,445}'::int[] & '{1623,623}';
+SELECT '{-1,3,1}'::int[] & '{1,2}';
 
 
 --test query_int