ICU: lock the mutex before calling an Unlocked function

The QIcuCodec::defaultCodecUnlocked function is not thread-safe
because it calls codecForNameUnlocked. In turn, that one accesses and
even modifies a QHash stored in the QCoreGlobalData singleton, which
is what makes it non-threadsafe.

In order to call the Unlocked function, we need to lock the mutex
first.

Change-Id: I915570110229f4c0929986aa26731244317ef6ab
Reviewed-by: Lars Knoll <lars.knoll@nokia.com>

src/corelib/codecs/qtextcodec.cpp

@@ -683,7 +683,9 @@ QTextCodec* QTextCodec::codecForLocale()
     QTextCodec *codec = globalData->codecForLocale.loadAcquire();
     if (!codec) {
 #ifdef QT_USE_ICU
+        textCodecsMutex()->lock();
         codec = QIcuCodec::defaultCodecUnlocked();
+        textCodecsMutex()->unlock();
 #else
         // setupLocaleMapper locks as necessary
         codec = setupLocaleMapper();