QString: fix UB in replace()

Comparing with <, >, <= or >= such pointers as are not pointing into
the same array is UB. A clever compiler could look at the code,
determine that the only valid execution is for it to return true, and
just always take the copy.

While that would be benign, it's not guaranteed that this would be the
outcome (it's UB, after all), and, of course, we don't want to take
the performance hit if we don't need it.

Pick-to: 5.15
Change-Id: I48cda232ff10a3c9fd4babcd7e7103a3aed126e8
Reviewed-by: Ville Voutilainen <ville.voutilainen@qt.io>

src/corelib/text/qstring.cpp

@@ -3038,7 +3038,8 @@ QChar *textCopy(const QChar *start, int len)
 bool pointsIntoRange(const QChar *ptr, const ushort *base, int len)
 {
     const QChar *const start = reinterpret_cast<const QChar *>(base);
-    return start <= ptr && ptr < start + len;
+    const std::less<const QChar *> less;
+    return !less(ptr, start) && less(ptr, start + len);
 }
 } // end namespace