iio: backend: fix out-of-bound write

iio: backend: fix out-of-bound write

The buffer is set to 80 character. If a caller write more characters,
count is truncated to the max available space in "simple_write_to_buffer".
But afterwards a string terminator is written to the buffer at offset count
without boundary check. The zero termination is written OUT-OF-BOUND.

Add a check that the given buffer is smaller then the buffer to prevent.

Fixes: 035b4989211d ("iio: backend: make sure to NULL terminate stack buffer")
Signed-off-by: Markus Burri <markus.burri@mt.com>
Reviewed-by: Nuno Sá <nuno.sa@analog.com>
Link: https://patch.msgid.link/20250508130612.82270-2-markus.burri@mt.com
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>