media: em28xx: Fix use-after-free in em28xx_alloc_urbs

When kzalloc() fails, em28xx_uninit_usb_xfer() will free
usb_bufs->buf and set it to NULL. Thus the later access
to usb_bufs->buf[i] will lead to null pointer dereference.
Also the kfree(usb_bufs->buf) after that is redundant.

Fixes: d571b592 ("media: em28xx: don't use coherent buffer for DMA transfers")
Signed-off-by: Dinghao Liu <dinghao.liu@zju.edu.cn>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>