media: vidtv: Check for null return of vzalloc

As the possible failure of the vzalloc(), e->encoder_buf might be NULL.
Therefore, it should be better to check it in order
to guarantee the success of the initialization.
If fails, we need to free not only 'e' but also 'e->name'.
Also, if the allocation for ctx fails, we need to free 'e->encoder_buf'
else.

Fixes: f90cf6079bf6 ("media: vidtv: add a bridge driver")
Signed-off-by: Jiasheng Jiang <jiasheng@iscas.ac.cn>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>
