fix check idx index

patch.go

@@ -397,7 +397,9 @@ func (d *partialArray) add(key string, val *lazyNode) error {
 		}
 		idx = len(ary) - idx
 	}
-
+	if idx < 0 || idx >= len(ary) || idx > len(cur) {
+		return fmt.Errorf("Unable to access invalid index: %d", idx)
+	}
 	copy(ary[0:idx], cur[0:idx])
 	ary[idx] = val
 	copy(ary[idx+1:], cur[idx:])

patch_test.go

@@ -227,6 +227,10 @@ var BadCases = []BadCase{
 		`{ "name":{ "foo": "bat", "qux": "bum"}}`,
 		`[ { "op": "replace", "path": "/foo/bar", "value":"baz"}]`,
 	},
+	{
+		`{ "foo": ["bar"]}`,
+		`[ {"op": "add", "path": "/foo/2", "value": "bum"}]`,
+	},
 }
 
 func TestAllCases(t *testing.T) {