Use logical operator && instead of & in vacuumparallel.c.

Use logical operator && instead of & in vacuumparallel.c.

As such the current usage of & won't produce incorrect results but it
would be better to use && to short-circuit the evaluation of second
condition when the same is not required.

Author: Ranier Vilela
Reviewed-by: Tom Lane, Bharath Rupireddy
Backpatch-through: 15, where it was introduced
Discussion: https://postgr.es/m/CAEudQApL8QcoYwQuutkWKY_h7gBY8F0Xs34YKfc7-G0i83K_pw@mail.gmail.com