Add a regression test.

Author: Max Schaefer

ql/test/query-tests/Security/CWE-190/tst2.go

@@ -14,3 +14,10 @@ func test3(r io.Reader) {
 	data, _ := ioutil.ReadAll(r)
 	ignore(make([]byte, len(data)+1)) // NOT OK
 }
+
+func test4(r io.Reader, ws []io.Writer) {
+	mw := io.MultiWriter(ws...)
+	ignore(make([]error, len(ws)+1)) // OK
+	data, _ := ioutil.ReadAll(r)
+	mw.Write(data)
+}