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
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