Check for NULL in dmu_free_long_range_impl()

A NULL should never be passed as the dnode_t pointer to the function
dmu_free_long_range_impl(). Regardless, because we have a reported
occurrence of this let's add some error handling to catch this.
Better to report a reasonable error to caller than panic the system.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Issue #3445