[sg fromtree] net: lwm2m: Fix seg-faults if a socket was closed in the meantime

Our application (posix-naive/debug) ran into segmentaion faults,
because the socket was closed between the "wait for sockets" and the
second if in the first loop. Added the check also to line 690
and 720.
The other uses of sock_ctx[i] are already checked against NULL

Signed-off-by: Peter Tönz <[email protected]>
(cherry picked from commit 46a5912)

Author: Peter Tönz

subsys/net/lib/lwm2m/lwm2m_engine.c

@@ -5798,13 +5798,15 @@ static void socket_loop(void)
 		}
 
 		for (i = 0; i < sock_nfds; ++i) {
-			if (sys_slist_is_empty(&sock_ctx[i]->pending_sends)) {
+			if (sock_ctx[i] != NULL &&
+			    sys_slist_is_empty(&sock_ctx[i]->pending_sends)) {
 				next_retransmit = retransmit_request(sock_ctx[i], timestamp);
 				if (next_retransmit < timeout) {
 					timeout = next_retransmit;
 				}
 			}
-			if (sys_slist_is_empty(&sock_ctx[i]->pending_sends)) {
+			if (sock_ctx[i] != NULL &&
+			    sys_slist_is_empty(&sock_ctx[i]->pending_sends)) {
 				check_notifications(sock_ctx[i], timestamp);
 			}
 		}