Free external entity parser in external_entity_resetter() when done
Free external entity parser in external_entity_resetter() when done

Deals with some of the memory leaks found by AddressSanitizer in
the test suite (Issue #23)

libexpat/libexpat,libexpat/libexpat,libexpat/libexpat,libexpat/libexpat,libexpat/libexpat,libexpat/libexpat