Fix dereference after null check in enqueue_range

If the bp is NULL, we have a hole. However, when we build with
assertions, we will dereference bp when blkid == DMU_SPILL_BLKID. When
this happens on a hole, we will have a NULL pointer dereference.

Reported-by: Coverity (CID-1524670)
Reviewed-by: Damian Szuberski <szuberskidamian@gmail.com>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Richard Yao <richard.yao@alumni.stonybrook.edu>
Closes #14264