Fix for Mantis #26456.

symtable.pas, searchsym_in_helper:
  * check whether extendeddef is assigned as this function is called when parsing the extended type and it was not found (the second pass is to display a more accurate error message and was added some weeks ago)

+ added test


git-svn-id: http://svn.freepascal.org/svn/fpc/trunk@28242 3ad0048d-3df7-0310-abae-a5850022a9f2