Fix null-termination of system call on getting the full os version.
Fix null-termination of system call on getting the full os version.

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