tor_strtok_r_impl incompatible with strtok_r
tor_strtok_r_impl doesn't behave strtok_r in certain cases, eg:
On empty string, it should return NULL directly (rather than an empty string).
When one of the separators is at the end of the input string, there should be no extra token at the end (ie. if you split "howdy!" on "!", there should be a single token "howdy", not "howdy" and "").
When the input string consists exclusively of separator characters, there should be no tokens.