Initialize number_of_jobs in NewRestoreOptions

Now that we're checking that the number_of_jobs passed in isn't zero or
negative, we need to actually initialize number_of_jobs to '1' when it
isn't set.

Pointed out by Rushabh Lathia, though not his patch.

Discussion: https://postgr.es/m/CAGPqQf2u1T3J=ANhCw1CuvzqjD80oWvMg2-2wmfG08gCm9hhHA@mail.gmail.com