Avoid accessing freed memory and freeing already freed resources

Seen in smoketest in a 4.2-based branch, but might perhaps happen in master
too. Should not hurt in any case to be more careful. ImplFreeSalGDI() is
called both from SalAbort() and from DestroySalInstance().

Change-Id: Ifac37e8ecfb432848bd3451dbda8634c59a06fba