Fix a memory leak spotted by Blaise Thorn

pdecsub.pas, parse_proc_head.consume_generic_type_parameter:
  * avoid a memory leak if there are no generic type parameters following

git-svn-id: http://svn.freepascal.org/svn/fpc/trunk@23543 3ad0048d-3df7-0310-abae-a5850022a9f2