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

src/backend/commands/vacuumparallel.c

@@ -612,7 +612,7 @@ parallel_vacuum_process_all_indexes(ParallelVacuumState *pvs, int num_index_scan
 		Assert(indstats->status == PARALLEL_INDVAC_STATUS_INITIAL);
 		indstats->status = new_status;
 		indstats->parallel_workers_can_process =
-			(pvs->will_parallel_vacuum[i] &
+			(pvs->will_parallel_vacuum[i] &&
 			 parallel_vacuum_index_is_parallel_safe(pvs->indrels[i],
 													num_index_scans,
 													vacuum));