std: fix more nilness findings

(found with x/tools/go/analysis/passes/nilness)

Change-Id: I1bdc7811efbecea95608e634f894cb6c656e3a5b
Reviewed-on: https://go-review.googlesource.com/c/go/+/564221
Reviewed-by: Robert Griesemer <gri@google.com>
Reviewed-by: Michael Pratt <mpratt@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

src/go/types/stmt.go

@@ -836,7 +836,7 @@ func (check *Checker) rangeStmt(inner stmtContext, s *ast.RangeStmt) {
 	type identType = ast.Ident
 	identName := func(n *identType) string { return n.Name }
 	sKey, sValue := s.Key, s.Value
-	var sExtra ast.Expr = nil
+	var sExtra ast.Expr = nil // (used only in types2 fork)
 	isDef := s.Tok == token.DEFINE
 	rangeVar := s.X
 	noNewVarPos := inNode(s, s.TokPos)

src/os/exec/exec_test.go

@@ -1659,8 +1659,8 @@ func TestCancelErrors(t *testing.T) {
 		// This test should kill the child process after 1ms,
 		// To maximize compatibility with existing uses of exec.CommandContext, the
 		// resulting error should be an exec.ExitError without additional wrapping.
-		if ee, ok := err.(*exec.ExitError); !ok {
-			t.Errorf("Wait error = %v; want %T", err, *ee)
+		if _, ok := err.(*exec.ExitError); !ok {
+			t.Errorf("Wait error = %v; want *exec.ExitError", err)
 		}
 	})
 

src/runtime/internal/wasitest/tcpecho_test.go

@@ -68,18 +68,14 @@ func TestTCPEcho(t *testing.T) {
 	defer subProcess.Process.Kill()
 
 	var conn net.Conn
-	var err error
 	for {
+		var err error
 		conn, err = net.Dial("tcp", host)
 		if err == nil {
 			break
 		}
 		time.Sleep(500 * time.Millisecond)
 	}
-	if err != nil {
-		t.Log(b.String())
-		t.Fatal(err)
-	}
 	defer conn.Close()
 
 	payload := []byte("foobar")

src/runtime/trace2cpu.go

@@ -261,9 +261,7 @@ func traceCPUSample(gp *g, mp *m, pp *p, stk []uintptr) {
 	if gp != nil {
 		hdr[1] = gp.goid
 	}
-	if mp != nil {
-		hdr[2] = uint64(mp.procid)
-	}
+	hdr[2] = uint64(mp.procid)
 
 	// Allow only one writer at a time
 	for !trace.signalLock.CompareAndSwap(0, 1) {