[8] Please review fix for 7175183: [macosx] Objective-C exception thrown when switching monitor configuration

[Alexander Zuev]

Hello,

  please review my fix for CR 7175183: [macosx] Objective-C exception 
thrown when switching monitor configuration

  The NPE happens because of the getBackupSurface method call results 
in the resetting of the current surface to null. The idea of the fix is 
to check for it in both native and java methods before using it.

Bug description is: 
http://bugs.sun.com/bugdatabase/view_bug.do?bug_id=7175183
Webrev can be found here: 
http://cr.openjdk.java.net/~kizune/7175183/webrev.00

With best regards,
Alex