usb: typec: altmodes/displayport: fix pin_assignment_show

This patch fixes negative indexing of buf array in pin_assignment_show
when get_current_pin_assignments returns 0 i.e. no compatible pin
assignments are found.

BUG: KASAN: use-after-free in pin_assignment_show+0x26c/0x33c
...
Call trace:
dump_backtrace+0x110/0x204
dump_stack_lvl+0x84/0xbc
print_report+0x358/0x974
kasan_report+0x9c/0xfc
__do_kernel_fault+0xd4/0x2d4
do_bad_area+0x48/0x168
do_tag_check_fault+0x24/0x38
do_mem_abort+0x6c/0x14c
el1_abort+0x44/0x68
el1h_64_sync_handler+0x64/0xa4
el1h_64_sync+0x78/0x7c
pin_assignment_show+0x26c/0x33c
dev_attr_show+0x50/0xc0

Fixes: 0e3bb7d6 ("usb: typec: Add driver for DisplayPort alternate mode")
Cc: stable@vger.kernel.org
Signed-off-by: Badhri Jagan Sridharan <badhri@google.com>
Reviewed-by: Heikki Krogerus <heikki.krogerus@linux.intel.com>
Link: https://lore.kernel.org/r/20230508214443.893436-1-badhri@google.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/typec/altmodes/displayport.c

@@ -516,6 +516,10 @@ static ssize_t pin_assignment_show(struct device *dev,
 
 	mutex_unlock(&dp->lock);
 
+	/* get_current_pin_assignments can return 0 when no matching pin assignments are found */
+	if (len == 0)
+		len++;
+
 	buf[len - 1] = '\n';
 	return len;
 }