Avoid some crashes when importing a pool with corrupt metadata

• Skip invalid DVAs when importing pools in readonly mode (in addition to when the config is untrusted).

• Upon encountering a DVA with a null VDEV, fail gracefully instead of panicking with a NULL pointer dereference.

Reviewed-by: Pavel Zakharov <pavel.zakharov@delphix.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Steve Mokris <smokris@softpixel.com>
Closes #9022