[9] Review Request: 8042212 [macosx] Toolkit.getScreenResolution() can return incorrect resolution

[Sergey Bylokhov]

Hello.
 Please review the small fix for jdk 9. According to documentation 
CGDisplayScreenSize() can return 0 if display is invalid we should be 
ready for that.

Bug: https://bugs.openjdk.java.net/browse/JDK-8042212
Webrev can be found at: http://cr.openjdk.java.net/~serb/8042212/webrev.00

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Best regards, Sergey.