[media] atomisp: one char read beyond end of string

We should verify that "ix < max_len" before we test whether we have
reached the NUL terminator.

Fixes: a49d2536 ("staging/atomisp: Add support for the Intel IPU v2")

Reported-by: David Binderman <dcb314@hotmail.com>
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>