Handle uninitialized values

as reported when compiling with optimizations (-02). Initialize the
variables early since they were being initialized in their loops and
later checked for -1.

Technically couldn't have happened since for example, user_part_inx1
would only be set to -1 if max_backfill_job_per_user_part was set. And
user_part_inx is only checked later if max_backfill_job_per_user_part
is set.

Same thing for part_inx, max_backfill_job_per_part and user_inx,
max_backfill_job_per_user.