fix(combinations-replacement): return an empty list for a non-positive group size

combinationsReplacement recurses on x.slice(i, x.length), which at i = 0 is
as long as the input, so the only exit left is the k === 1 base case. For
k <= 0 the counter starts below that case and moves away from it, and the
recursion overflows the stack. The sibling combinations recurses on
x.slice(i + 1, x.length), runs out of input and returns [] for the same
arguments.

Guard the non-positive group size up front so both functions agree.

test/combinations_replacement.test.js

@@ -15,4 +15,27 @@ describe("combinations", function () {
             [2, 2]
         ]);
     });
+    it("returns no combinations when choosing zero at a time", function () {
+        assert.deepEqual(
+            ss.combinationsReplacement(Object.freeze([1, 2, 3]), 0),
+            []
+        );
+    });
+    it("returns no combinations for a negative group size", function () {
+        assert.deepEqual(
+            ss.combinationsReplacement(Object.freeze([1, 2, 3]), -1),
+            []
+        );
+    });
+    it("agrees with combinations on non-positive group sizes", function () {
+        const x = Object.freeze([1, 2, 3]);
+        assert.deepEqual(
+            ss.combinationsReplacement(x, 0),
+            ss.combinations(x, 0)
+        );
+        assert.deepEqual(
+            ss.combinationsReplacement(x, -1),
+            ss.combinations(x, -1)
+        );
+    });
 });