Input: hynitron_cstxxx - validate touch count and finger IDs

Input: hynitron_cstxxx - validate touch count and finger IDs

The driver allocates max_touch_num input slots, which are indexed from
zero through max_touch_num - 1. The current check allows a finger ID
equal to max_touch_num to reach cst3xx_report_contact(). While the input
core ignores out-of-range slot indices, reporting touch data without a
valid slot change corrupts the touch state of the previously active slot.

The touch count is read from the controller's report and is used to
index the fixed-size report buffer without first checking its range.
Reject counts larger than the supported number of touch slots before
checking the trailing byte or parsing touch data.

Reject finger IDs equal to or greater than max_touch_num, and return
immediately when an invalid finger ID is encountered so that corrupt
touch frames are discarded instead of reporting partial contact state.

The V821 Avaota F1 board configures the vendor driver with one touch
slot, so finger ID 1 is already invalid on that device.

Fixes: 66603243f528 ("Input: add driver for Hynitron cstxxx touchscreens")
Signed-off-by: Jianing Li <m13940358460@163.com>
Cc: stable@vger.kernel.org
Link: https://patch.msgid.link/20260804031339.2379-1-m13940358460@163.com
Assisted-by: Antigravity:gemini-3.6-flash
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>