Cause an error in PARSE for invalid args to TO (and THRU)

Previously, PARSE over string input would silently ignore invalid arguments to TO (and THRU). With this fix, invalid args to TO and THRU correctly cause an error:

    >> parse "foo" [to 1.2]
    ** Script error: PARSE - invalid rule or usage of rule: to
    ** Where: parse
    ** Near: parse "foo" [to 1.2]

The problem was identified via static code analysis with Cppcheck.