Don't lose precision in QMacTimeZonePrivate::previousTransition()

NSTimeInterval is a typedef for double, but the code stored its value in
an int, and only then multiplied by 1000.

Fix by only truncating NSTimeIntervals to int(64_t) *after* the
multiplication by 1e3 to get milliseconds.

While it's highly unlikely that a transition will have fractional seconds
length, don't assume if you can just calculate the more exact result.

Adapted-From: Marc Mutz <marc.mutz@kdab.com>
Change-Id: I0911b9c945a94ca24c3dfb23ed6a849141076326
Reviewed-by: Jake Petroules <jake.petroules@qt.io>

src/corelib/tools/qtimezoneprivate_mac.mm

@@ -295,7 +295,7 @@ QTimeZonePrivate::Data QMacTimeZonePrivate::previousTransition(qint64 beforeMSec
             break;
     }
     if (prevSecs < endSecs) // i.e. we did make it into that while loop
-        return data(qint64(prevSecs) * 1000);
+        return data(qint64(prevSecs * 1e3));
 
     // No transition data; or first transition later than requested time.
     return invalidData();