runtime: Fix panic when trying to stop CPU profiling with profiler turned off

Fixes #7063.

R=golang-codereviews, iant
CC=golang-codereviews
https://golang.org/cl/47950043

src/pkg/runtime/cpuprof.c

@@ -168,7 +168,7 @@ runtime·SetCPUProfileRate(intgo hz)
 		runtime·noteclear(&prof->wait);
 
 		runtime·setcpuprofilerate(tick, hz);
-	} else if(prof->on) {
+	} else if(prof != nil && prof->on) {
 		runtime·setcpuprofilerate(nil, 0);
 		prof->on = false;
 

src/pkg/runtime/runtime_test.go

@@ -126,3 +126,8 @@ func TestRuntimeGogoBytes(t *testing.T) {
 
 	t.Fatalf("go tool nm did not report size for runtime.gogo")
 }
+
+// golang.org/issue/7063
+func TestStopCPUProfilingWithProfilerOff(t *testing.T) {
+	SetCPUProfileRate(0)
+}