parse.y: pack op_tbl

* parse.y (op_tbl): pack limited size strings.  gcc/clang warn
  if initializer-string for char array is too long, but no option
  to err it now.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@46688 b2dd03c8-39d4-4d8f-98ff-823fe69b080e