ada-tasks.c::read_atcb: start from a cleared ada_task_info result

ada-tasks.c::read_atcb: start from a cleared ada_task_info result

The purpose of this patch is not to fix a bug per se, but rather
to robustify this function to make sure it never returns a struct
ada_task_info where some of the fields are left uninitialized.
Reading the current implementation, it attempts to methodically
set them all one by one: but it's not excluded that a future
change might miss something. A memset is cheap and make sure that
this function returns repeatable results.

This in turns allows us to remove some assignments which have become
redundant.

gdb/ChangeLog:

        * ada-tasks.c (read_atcb): Clear task_info before computing
        the value of each of its fields.