[Race] Fix datarace in GetLevel

`std.Level` is protected by mutex in setter (SetLevel),
so it must be protected in geetter (GetLevel) too.

Signed-off-by: Anton Tiurin <noxiouz@yandex.ru>

exported.go

@@ -36,6 +36,8 @@ func SetLevel(level Level) {
 
 // GetLevel returns the standard logger level.
 func GetLevel() Level {
+	std.mu.Lock()
+	defer std.mu.Unlock()
 	return std.Level
 }