runtime: fix go of nil func value

Currently runtime derefences nil with m->locks>0,
which causes unrecoverable fatal error.
Panic instead.
Fixes #8045.

LGTM=rsc
R=golang-codereviews, rsc
CC=golang-codereviews, khr
https://golang.org/cl/97620043

src/pkg/runtime/crash_test.go

@@ -158,6 +158,14 @@ func TestGoexitCrash(t *testing.T) {
 	}
 }
 
+func TestGoNil(t *testing.T) {
+	output := executeTest(t, goNilSource, nil)
+	want := "go of nil func value"
+	if !strings.Contains(output, want) {
+		t.Fatalf("output:\n%s\n\nwant output containing: %s", output, want)
+	}
+}
+
 const crashSource = `
 package main
 
@@ -343,3 +351,16 @@ func main() {
 	runtime.Goexit()
 }
 `
+
+const goNilSource = `
+package main
+
+func main() {
+	defer func() {
+		recover()
+	}()
+	var f func()
+	go f()
+	select{}
+}
+`

src/pkg/runtime/proc.c

@@ -1816,6 +1816,10 @@ runtime·newproc1(FuncVal *fn, byte *argp, int32 narg, int32 nret, void *callerp
 	int32 siz;
 
 //runtime·printf("newproc1 %p %p narg=%d nret=%d\n", fn->fn, argp, narg, nret);
+	if(fn == nil) {
+		m->throwing = -1;  // do not dump full stacks
+		runtime·throw("go of nil func value");
+	}
 	m->locks++;  // disable preemption because it can be holding p in a local var
 	siz = narg + nret;
 	siz = (siz+7) & ~7;