fix MWEReader's get_next_token -- return token clones as "the caller must dispose of the Token it receives" (according to Corpus2 API docs)

fix match token-level condition: immediately terminate and return empty match result when curr pos is outside sent

This is a workaround for lovely boost 1.48 incompatibility causing compile errors on assignment to StrSet from a temporary StrSet, see https://svn.boost.org/trac/boost/ticket/6167 for more details.