QDateTime: fix UB (signed overflow) in addDays()

The comment indicated that the author expected any overflow to be
caught by a bounds check in the subsequent function, however, signed
overflow is UB, so anything can happen.

Fix by using our API for safe additions instead.

Pick-to: 6.3 6.2 5.15
Change-Id: I41909defffa5305b02fdfcf6d5808e0d9fd5924f
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/time/qdatetime.cpp

@@ -1227,9 +1227,10 @@ QDate QDate::addDays(qint64 ndays) const
     if (isNull())
         return QDate();
 
-    // Due to limits on minJd() and maxJd() we know that any overflow
-    // will be invalid and caught by fromJulianDay().
-    return fromJulianDay(jd + ndays);
+    if (qint64 r; Q_UNLIKELY(qAddOverflow(jd, ndays, &r)))
+        return QDate();
+    else
+        return fromJulianDay(r);
 }
 
 /*!