[media] v4l2-mem2mem: no need to initialize b in v4l2_m2m_next_buf and v4l2_m2m_buf_remove

The first time b is used, it is assigned to the first element of the rdy_queue
list. There is no need to set it to NULL before.

Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: Kamil Debski <k.debski@samsung.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>