STM32: optimized clock instance access

libuavcan_drivers/stm32/driver/src/uc_stm32_clock.cpp

@@ -405,8 +405,6 @@ void setUtcSyncParams(const UtcSyncParams& params)
 
 SystemClock& SystemClock::instance()
 {
-    MutexLocker mlocker(clock::mutex);
-
     static union SystemClockStorage
     {
         uavcan::uint8_t buffer[sizeof(SystemClock)];
@@ -418,6 +416,7 @@ SystemClock& SystemClock::instance()
 
     if (!clock::initialized)
     {
+        MutexLocker mlocker(clock::mutex);
         clock::init();
         new (ptr)SystemClock();
     }