Add defensive assertion to vdev_queue_aggregate()

a6ccb36b948efed0eeee4fcf99fe4b5fb81ae1d5 had been intended to include
this to silence Coverity reports, but this one was missed by mistake.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Richard Yao <richard.yao@alumni.stonybrook.edu>
Closes #14043