internal/counter: avoid nil check panic

Some tests in this package attempt to close/unmmap files
by calling a test helper function 'close'.
When telemetry is off, mapped file, not the file, can be nil.
Actually, the file argument (f) of close is never nil.

Updates golang/go#60967

Change-Id: I4ca3feb233ede3a6512a4f3d50460c2fb28a6559
Reviewed-on: https://go-review.googlesource.com/c/telemetry/+/505720
TryBot-Result: Gopher Robot <[email protected]>
Run-TryBot: Hyang-Ah Hana Kim <[email protected]>
Reviewed-by: Jamal Carvalho <[email protected]>

Author: hyangah

internal/counter/counter_test.go

@@ -78,12 +78,13 @@ func TestBasic(t *testing.T) {
 // this is needed in Windows so that the generated testing.go file
 // can clean up the temporary test directory
 func close(f *file) {
-	if f == nil {
+	mf := f.current.Load()
+	if mf == nil {
 		// telemetry might have been off
 		return
 	}
-	f.current.Load().f.Close()
-	mmap.Munmap(f.current.Load().mapping)
+	mmap.Munmap(mf.mapping)
+	mf.f.Close()
 }
 
 func TestLarge(t *testing.T) {