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/runtime_test.go

diff --git a/src/pkg/runtime/runtime_test.go b/src/pkg/runtime/runtime_test.go
index f6b48ba..c673275 100644
--- a/src/pkg/runtime/runtime_test.go
+++ b/src/pkg/runtime/runtime_test.go
@@ -126,3 +126,8 @@
 
 	t.Fatalf("go tool nm did not report size for runtime.gogo")
 }
+
+// golang.org/issue/7063
+func TestStopCPUProfilingWithProfilerOff(t *testing.T) {
+	SetCPUProfileRate(0)
+}