Fix potential crash in PFOfflineObjectEncoder.
Fix potential crash in PFOfflineObjectEncoder.

ParsePlatform/Parse-SDK-iOS-OSX,natanrolnik/Parse-SDK-iOS-OSX,natanrolnik/Parse-SDK-iOS-OSX,podkovyrin/Parse-SDK-iOS-OSX,gonzalonunez/Parse-SDK-iOS-OSX,podkovyrin/Parse-SDK-iOS-OSX,gonzalonunez/Parse-SDK-iOS-OSX,ParsePlatform/Parse-SDK-iOS-OSX