net: l2: ppp: ensure proper null-termination in `ppp_fsm_close`

- Remove the `strlen`, as it is unnecessary for safe copying.
- Limit the `strncpy` to `sizeof(fsm->terminate_reason) - 1`
  to ensure the last byte is reserved for null-termination.
- Add an explicit null terminator to guarantee correct string termination.

Signed-off-by: Pisit Sawangvonganan <pisit@ndrsolution.com>

subsys/net/l2/ppp/fsm.c

diff --git a/subsys/net/l2/ppp/fsm.c b/subsys/net/l2/ppp/fsm.c
index 1f64de1..3dd2802 100644
--- a/subsys/net/l2/ppp/fsm.c
+++ b/subsys/net/l2/ppp/fsm.c
@@ -226,10 +226,10 @@
 	case PPP_OPENED:
 	case PPP_REQUEST_SENT:
 		if (reason) {
-			int len = strlen(reason);
+			int limit_len = sizeof(fsm->terminate_reason) - 1;
 
-			len = MIN(sizeof(fsm->terminate_reason) - 1, len);
-			strncpy(fsm->terminate_reason, reason, len);
+			strncpy(fsm->terminate_reason, reason, limit_len);
+			fsm->terminate_reason[limit_len] = '\0';
 		}
 
 		terminate(fsm, PPP_CLOSING);