QVarLengthArray: fix aliasing error in insert(it, n, v)

Taking the copy after the resize is completely pointless: the copy is
there to ensure that `t`, being a reference potentially aliasing an
element in [begin(), end()[ before the resize(), isn't invalidated by
the resize(), so it must be taken before resize().

Add a comment so the next rewrite doesn't cause this to be mixed up
again.

[ChangeLog][QtCore][QVarLengthArray] Fixed an aliasing bug affecting
insertions of objects aliasing existing elements.

Pick-to: 6.2 6.1 6.0 5.15 5.12
Change-Id: I26bc449fa99bf8d09a19147a12a69ac4314cc61d
Reviewed-by: Giuseppe D'Angelo <giuseppe.dangelo@kdab.com>

src/corelib/tools/qvarlengtharray.h

@@ -647,8 +647,8 @@ Q_OUTOFLINE_TEMPLATE typename QVarLengthArray<T, Prealloc>::iterator QVarLengthA
 
     qsizetype offset = qsizetype(before - ptr);
     if (n != 0) {
+        const T copy(t); // `t` could alias an element in [begin(), end()[
         resize(s + n);
-        const T copy(t);
         if (!QTypeInfo<T>::isRelocatable) {
             T *b = ptr + offset;
             T *j = ptr + s;