QVarLengthArray: fix UB (precondition violation) in range-erase()

When the range-erase() function is called with an empty, valid range,
it passed equal iterators to both the first and the last arguments of
std::move(f, l, d) (the algorithm, not the cast). This is UB, since it
violates the algorithm's precondition that d ∉ [f,l). For non-empty
ranges, f > d, thus d < f, hence d ∉ [f,l), so everything is ok
_there_.

Fix the empty range case by returning early.

Reviewers may question the precondition, expecting that std::move(f,
l, f) just be a no-op, but the algorithm, as specified, performs
self-move-assignments in that case, which need not be supported. In
fact, the Hinnant criterion, itself only applicable if one calls for
self-swap-safety, asks for self-move-assignment-safety only for
objects in the moved-from state. QVarLengthArray, itself, meets only
the Hinnant criterion; self-move-assignment of non-empty QVLAs invokes
UB.

So, the UB here is real.

Qt 5.15 uses std::copy() here, which has the same precondition as
std::move(), and the same fix applies there, too.

[ChangeLog][QtCore][QVarLengthArray] Fixed a bug where range-erase()
could invoke undefined behavior when called with an empty range.

Pick-to: 6.2 5.15
Change-Id: I656aa09d025168d6d9ef088ad4c6954d216f0d54
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/tools/qvarlengtharray.h

@@ -682,6 +682,9 @@ Q_OUTOFLINE_TEMPLATE typename QVarLengthArray<T, Prealloc>::iterator QVarLengthA
     qsizetype l = qsizetype(aend - cbegin());
     qsizetype n = l - f;
 
+    if (n == 0) // avoid UB in std::move() below
+        return data() + f;
+
     if constexpr (QTypeInfo<T>::isComplex) {
         std::move(begin() + l, end(), QT_MAKE_CHECKED_ARRAY_ITERATOR(begin() + f, size() - f));
         std::destroy(end() - n, end());