go/analysis/passes/testinggoroutine: fix panic in goStmtFun

For function declared in other files, the identifier denoted function
will be nil, cause the analysis panics. To fix this, we just skip that
identifier for now, until golang/go#48141 is resolved.

Fixes golang/go#48124

Change-Id: I87876505ee5964639ed3d1772d541c00d091ceb6
Reviewed-on: https://go-review.googlesource.com/c/tools/+/347089
Trust: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Tim King <taking@google.com>
gopls-CI: kokoro <noreply+kokoro@google.com>

go/analysis/passes/testinggoroutine/testdata/src/a/a.go

@@ -53,6 +53,8 @@ func f(t *testing.T, _ string) {
 	t.Fatal("TestFailed")
 }
 
+func g() {}
+
 func TestBadFatalIssue47470(t *testing.T) {
 	go f(t, "failed test 1") // want "call to .+T.+Fatal from a non-test goroutine"
 
@@ -272,3 +274,7 @@ func TestWithCustomType(t *testing.T) {
 		}(i)
 	}
 }
+
+func TestIssue48124(t *testing.T) {
+	go h()
+}

go/analysis/passes/testinggoroutine/testdata/src/a/b.go

@@ -0,0 +1,7 @@
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package a
+
+func h() {}

go/analysis/passes/testinggoroutine/testinggoroutine.go

@@ -127,6 +127,10 @@ func goStmtFun(goStmt *ast.GoStmt) ast.Node {
 	switch goStmt.Call.Fun.(type) {
 	case *ast.Ident:
 		id := goStmt.Call.Fun.(*ast.Ident)
+		// TODO(cuonglm): improve this once golang/go#48141 resolved.
+		if id.Obj == nil {
+			break
+		}
 		if funDecl, ok := id.Obj.Decl.(ast.Node); ok {
 			return funDecl
 		}