[media] cx18: check for allocation failure in cx18_read_eeprom()

It upsets static checkers when we don't check for allocation failure.  I
moved the memset() of "tv" earlier so we don't use uninitialized data on
error.
Fixes: 1d212cf0 ('[media] cx18: struct i2c_client is too big for stack')

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Andy Walls <awalls@md.metrocast.net>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>