runtime: don't assume that _ = *s will panic if s is nil

    
    With the gc toolchain apparently
            var s *string
            _ = *s
    is enough to panic with a nil pointer dereference. The gccgo compiler
    will simply discard the dereference, which I think is a reasonable and
    acceptable optimization. Change the tests to use an exported variable
    instead. The tests are not currently run, but they will be with a
    later patch to gotools.
    
    Reviewed-on: https://go-review.googlesource.com/46450

From-SVN: r249562

gcc/go/gofrontend/MERGE

-b5c9fe259ec43f8079581c3bea0f1d12d85213a7
+8804c912363320e0c229c5a471fb6f4b9e5965e6
 
 The first line of this file holds the git revision number of the last
 merge done from the gofrontend repository.

libgo/go/runtime/testdata/testprog/crash.go

@@ -13,6 +13,8 @@ func init() {
 	register("Crash", Crash)
 }
 
+var NilPointer *string
+
 func test(name string) {
 	defer func() {
 		if x := recover(); x != nil {
@@ -21,8 +23,7 @@ func test(name string) {
 		fmt.Printf(" done\n")
 	}()
 	fmt.Printf("%s:", name)
-	var s *string
-	_ = *s
+	*NilPointer = name
 	fmt.Print("SHOULD NOT BE HERE")
 }
 

libgo/go/runtime/testdata/testprogcgo/crash.go

@@ -13,6 +13,8 @@ func init() {
 	register("Crash", Crash)
 }
 
+var NilPointer *string
+
 func test(name string) {
 	defer func() {
 		if x := recover(); x != nil {
@@ -21,8 +23,7 @@ func test(name string) {
 		fmt.Printf(" done\n")
 	}()
 	fmt.Printf("%s:", name)
-	var s *string
-	_ = *s
+	*NilPointer = name
 	fmt.Print("SHOULD NOT BE HERE")
 }