accel/ivpu: Use effective buffer size for zero terminator

Use the effective written size instead of original size as index for zero
termination. If the input from user-space is to larger and the input is
truncated, the original size is out-of-bound.
Since there is an upfront size check here, the change is for consistency.

Signed-off-by: Markus Burri <markus.burri@mt.com>
Reviewed-by: Jacek Lawrynowicz <jacek.lawrynowicz@linux.intel.com>
Signed-off-by: Jacek Lawrynowicz <jacek.lawrynowicz@linux.intel.com>
Link: https://lore.kernel.org/r/20250508130612.82270-3-markus.burri@mt.com