parse.y: clear lex_strterm

* parse.y (parser_heredoc_restore): clear lex_strterm always to get
  rid of marking recycled node.  this bug is revealed by r41372 with
  GC.stress=true.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@41378 b2dd03c8-39d4-4d8f-98ff-823fe69b080e