Revert "QMutexPool: avoid QVarLengthArray of QAtomicPointers"

This reverts commit 4579d966af.

This causes a miscompilation with ICC 16 on Windows (MSVC 2015 ABI): the
vector created by new[] with () in:

      mutexes(new QAtomicPointer<QMutex>[size]()), // (): zero-initialize

does not actually zero-initialize (see disassembly in the bug
report). This is definitely a compiler bug.

Since we plan on removing QMutexPool in Qt 5.8 anyway, let's just revert
the patch.

Task-number: QTBUG-53360
Change-Id: I06bae9392f534e45b3f1ffff144e823b747e7962
Reviewed-by: Kai Koehne <kai.koehne@qt.io>

src/corelib/thread/qmutexpool.cpp

@@ -92,10 +92,11 @@ Q_GLOBAL_STATIC_WITH_ARGS(QMutexPool, globalMutexPool, (QMutex::Recursive))
     QMutexPool is destructed.
 */
 QMutexPool::QMutexPool(QMutex::RecursionMode recursionMode, int size)
-    : count(size),
-      mutexes(new QAtomicPointer<QMutex>[size]()), // (): zero-initialize
-      recursionMode(recursionMode)
+    : mutexes(size), recursionMode(recursionMode)
 {
+    for (int index = 0; index < mutexes.count(); ++index) {
+        mutexes[index].store(0);
+    }
 }
 
 /*!
@@ -104,8 +105,8 @@ QMutexPool::QMutexPool(QMutex::RecursionMode recursionMode, int size)
 */
 QMutexPool::~QMutexPool()
 {
-    qDeleteAll(mutexes, mutexes + count);
-    delete[] mutexes;
+    for (int index = 0; index < mutexes.count(); ++index)
+        delete mutexes[index].load();
 }
 
 /*!

src/corelib/thread/qmutexpool_p.h

@@ -66,7 +66,7 @@ public:
     ~QMutexPool();
 
     inline QMutex *get(const void *address) {
-        int index = uint(quintptr(address)) % count;
+        int index = uint(quintptr(address)) % mutexes.count();
         QMutex *m = mutexes[index].load();
         if (m)
             return m;
@@ -78,8 +78,7 @@ public:
 
 private:
     QMutex *createMutex(int index);
-    int count;
-    QAtomicPointer<QMutex> *mutexes;
+    QVarLengthArray<QAtomicPointer<QMutex>, 131> mutexes;
     QMutex::RecursionMode recursionMode;
 };