cmd/compile: only check return for valid functions

CheckReturn uses fn.Type() unconditionally, so for invalid function,
fn.Type() will be nil, causes the compiler crashes.

Updates #43311

Change-Id: I4420dd296c72ea83986b38fbf2c7f51fa59757c8
Reviewed-on: https://go-review.googlesource.com/c/go/+/298709
Trust: Cuong Manh Le <[email protected]>
Run-TryBot: Cuong Manh Le <[email protected]>
TryBot-Result: Go Bot <[email protected]>
Reviewed-by: Matthew Dempsky <[email protected]>

Author: cuonglm

src/cmd/compile/internal/typecheck/typecheck.go

@@ -2104,7 +2104,7 @@ func CheckUnused(fn *ir.Func) {
 
 // CheckReturn makes sure that fn terminates appropriately.
 func CheckReturn(fn *ir.Func) {
-	if fn.Type().NumResults() != 0 && len(fn.Body) != 0 {
+	if fn.Type() != nil && fn.Type().NumResults() != 0 && len(fn.Body) != 0 {
 		markBreak(fn)
 		if !isTermNodes(fn.Body) {
 			base.ErrorfAt(fn.Endlineno, "missing return at end of function")

test/fixedbugs/issue17588.go

@@ -1,4 +1,4 @@
-// errorcheck
+// errorcheck -d=panic
 
 // Copyright 2016 The Go Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style
@@ -11,10 +11,10 @@
 
 package p
 
-type F func(b T)  // ERROR "T .*is not a type|expected type"
+type F func(b T) // ERROR "T .*is not a type|expected type"
 
 func T(fn F) {
-    func() {
-        fn(nil)  // If Decldepth is not initialized properly, typecheckclosure() Fatals here.
-    }()
+	func() {
+		fn(nil) // If Decldepth is not initialized properly, typecheckclosure() Fatals here.
+	}()
 }