QString: optimize insert() of a substring

The old code malloc()ed a buffer to hold a copy of the data if a
substring of *this was to be inserted. Instead, use a QVarLengthArray.

Pick-to: 5.15
Change-Id: Ia3b4d7509bff2375ec0da5c890ecff2e9f7f335c
Reviewed-by: Volker Hilsheimer <volker.hilsheimer@qt.io>

src/corelib/text/qstring.cpp

@@ -2578,11 +2578,8 @@ QString& QString::insert(int i, const QChar *unicode, int size)
     const std::less<const ushort*> less;
     if (!less(s, d.data()) && less(s, d.data() + d.size)) {
         // Part of me - take a copy
-        ushort *tmp = static_cast<ushort *>(::malloc(size * sizeof(QChar)));
-        Q_CHECK_PTR(tmp);
-        memcpy(tmp, s, size * sizeof(QChar));
-        insert(i, reinterpret_cast<const QChar *>(tmp), size);
-        ::free(tmp);
+        const QVarLengthArray<ushort> copy(s, s + size);
+        insert(i, reinterpret_cast<const QChar *>(copy.data()), size);
         return *this;
     }