Fix for Mantis #24903.

ncon.pas, tstringconstnode:
  * buildderefimpl: don't build the deref if we don't need to
  * derefimpl: do not try to resolve the deref if it wasn't stored at all



git-svn-id: http://svn.freepascal.org/svn/fpc/trunk@25329 3ad0048d-3df7-0310-abae-a5850022a9f2