vb2: core: Skip planes array verification if pb is NULL

vb2: core: Skip planes array verification if pb is NULL

commit 126f40298446a82116e1f92a1aaf72b8c8228fae upstream.

An earlier patch fixing an input validation issue introduced another
issue: vb2_core_dqbuf() is called with pb argument value NULL in some
cases, causing a NULL pointer dereference. Fix this by skipping the
verification as there's nothing to verify.

Fixes: e7e0c3e26587 ("[media] videobuf2-core: Check user space planes array in dqbuf")
Signed-off-by: David R <david@unsolicited.net>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Reviewed-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>