The field fts_token->position is not initialized in
row_merge_fts_doc_tokenize(). We cannot have that field
without changing the fulltext parser plugin ABI
(adding st_mysql_ftparser_boolean_info::position,
as it was done in MySQL 5.7 in WL#6943).

The InnoDB fulltext parser plugins "ngram" and "Mecab" that were
introduced in MySQL 5.7 do depend on that field. But the simple_parser
does not. Apparently, simple_parser is leaving the field as 0.

So, in our fix we will assume that the missing position field is 0.