QFreeList: replace a Q_ASSERT(false) with Q_UNREACHABLE()

Q_ASSERT goes away in release builds, Q_UNREACHABLE() does not.

This also solves the GCC 12 warning about out-of-bounds access in
QAbstractEventDispatcherPrivate::allocateTimerId():

In member function ‘_PTp* std::__atomic_base<_PTp*>::load(std::memory_order) const [with _PTp = QFreeListElement<void>]’,
    inlined from ‘_Tp* std::atomic<_Tp*>::load(std::memory_order) const [with _Tp = QFreeListElement<void>]’ at atomic:579:25,
    inlined from ‘static T QAtomicOps<X>::loadAcquire(const std::atomic<T>&) [with T = QFreeListElement<void>*; X = QFreeListElement<void>*]’ at thread/qatomic_cxx11.h:249:29,
    inlined from ‘X* QBasicAtomicPointer<X>::loadAcquire() const [with X = QFreeListElement<void>]’ at thread/qbasicatomic.h:233:64,
    inlined from ‘int QFreeList<T, ConstantsType>::next() [with T = void; ConstantsType = QtTimerIdFreeListConstants]’ at qfreelist_p.h:245:34,
    inlined from ‘static int QAbstractEventDispatcherPrivate::allocateTimerId()’ at kernel/qabstracteventdispatcher.cpp:99:24:
bits/atomic_base.h:820:31: warning: ‘long unsigned int __atomic_load_8(const volatile void*, int)’ writing 8 bytes into a region of size 0 overflows the destination [-Wstringop-overflow=]
  820 |         return __atomic_load_n(&_M_p, int(__m));
      |                ~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~
In file included from qglobalstatic.h:1,
                 from qglobal.h:1395:
qglobalstatic.h: In static member function ‘static int QAbstractEventDispatcherPrivate::allocateTimerId()’:
qglobalstatic.h:127:23: note: at offset -8 into destination object ‘holder’ of size 56
  127 |         static Holder holder;
      |                       ^~~~~~

Pick-to: 5.15 6.2 6.3
Change-Id: I74249c52dc02478ba93cfffd16d232b275d5d216
Reviewed-by: Marc Mutz <marc.mutz@qt.io>

src/corelib/tools/qfreelist_p.h

@@ -160,7 +160,7 @@ class QFreeList
                 return i;
             x -= size;
         }
-        Q_ASSERT(false);
+        Q_UNREACHABLE();
         return -1;
     }