cppcheck: (warning) Possible null pointer dereference: dnp

The dnp argument can only be set to NULL when the DNODE_DRY_RUN flag
is set. In which case, an early return path will be executed and a
NULL pointer dereference at the given location is impossible. Add
an additional ASSERT to silence the cppcheck warning and document
that dbp must never be NULL at the point in the function.

[module/zfs/dnode.c:1566]: (warning) Possible null pointer deref: dnp

Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #9732