Fix for Mantis #26271: Don't trash local variables that are used for the Default() intrinsic.

* ngenutil.pas, tnodeutils.trashable_sym:
    check for vo_is_default_var flag and don't consider such as trashable

+ added test

git-svn-id: http://svn.freepascal.org/svn/fpc/trunk@27872 3ad0048d-3df7-0310-abae-a5850022a9f2