The first time through the loop prevdb and prevhdl are NULL. They
are then both set, but only prevdb is checked. Add an ASSERT to
make it clear that prevhdl must be set when prevdb is.
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Kleber <klebertarcisio@yahoo.com.br>
Closes#10754Closes#11575