NSData: fix an errno stomp
NSData: fix an errno stomp

We would attempt to remove the temporary file.  However, that will
result in calls which can destroy the original errno.  Save the previous
errno before performing the cleanup.

e78l/swift-corelibs-foundation,e78l/swift-corelibs-foundation,e78l/swift-corelibs-foundation,e78l/swift-corelibs-foundation,parkera/swift-corelibs-foundation,parkera/swift-corelibs-foundation,apple/swift-corelibs-foundation,apple/swift-corelibs-foundation,parkera/swift-corelibs-foundation,parkera/swift-corelibs-foundation,apple/swift-corelibs-foundation,parkera/swift-corelibs-foundation,e78l/swift-corelibs-foundation,apple/swift-corelibs-foundation,apple/swift-corelibs-foundation