tests: net: socket: udp: Don't call memset() on a NULL pointer

cmsgbuf pointer can be NULL, therefore verify that before calling
memset() on it.

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>

tests/net/socket/udp/src/main.c

@@ -1495,7 +1495,9 @@ static void comm_sendmsg_recvmsg(int client_sock,
 	io_vector[0].iov_len = sizeof(buf);
 
 	memset(msg, 0, sizeof(*msg));
-	memset(cmsgbuf, 0, cmsgbuf_len);
+	if (cmsgbuf != NULL) {
+		memset(cmsgbuf, 0, cmsgbuf_len);
+	}
 	msg->msg_control = cmsgbuf;
 	msg->msg_controllen = cmsgbuf_len;
 	msg->msg_iov = io_vector;
@@ -1572,7 +1574,9 @@ static void comm_sendmsg_recvmsg(int client_sock,
 	io_vector[1].iov_len = sizeof(buf);
 
 	memset(msg, 0, sizeof(*msg));
-	memset(cmsgbuf, 0, cmsgbuf_len);
+	if (cmsgbuf != NULL) {
+		memset(cmsgbuf, 0, cmsgbuf_len);
+	}
 	msg->msg_control = cmsgbuf;
 	msg->msg_controllen = cmsgbuf_len;
 	msg->msg_iov = io_vector;
@@ -1642,7 +1646,9 @@ static void comm_sendmsg_recvmsg(int client_sock,
 	io_vector[0].iov_len = sizeof(buf2);
 
 	memset(msg, 0, sizeof(*msg));
-	memset(cmsgbuf, 0, cmsgbuf_len);
+	if (cmsgbuf != NULL) {
+		memset(cmsgbuf, 0, cmsgbuf_len);
+	}
 	msg->msg_control = cmsgbuf;
 	msg->msg_controllen = cmsgbuf_len;
 	msg->msg_iov = io_vector;