The problem was in and_all_keys(), the code of MDEV-9759 which calculates
the new tree weight:

First, it didn't take into account the case when

(next->next_key_part=tmp) == NULL

and dereferenced a NULL pointer when getting tmp->weight.

Second, "if (param->alloced_sel_args > SEL_ARG::MAX_SEL_ARGS) break"
could leave the loop with incorrect value of weight.

Fixed by introducing SEL_ARG::update_weight_locally() and calling it
at the end of the function. This allows to avoid caring about all the
above cases.