Fix of undefined behavior of null referencing (netty#10016)

Motivation:

Current code depends on some "undefined behaviour".

Modification:

Fix of undefined behavior of null referencing

Result:

Correct c code.

transport-native-unix-common/src/main/c/netty_unix_socket.c

@@ -16,6 +16,7 @@
 #include <fcntl.h>
 #include <errno.h>
 #include <unistd.h>
+#include <stddef.h>
 #include <stdint.h>
 #include <stdlib.h>
 #include <string.h>
@@ -50,7 +51,7 @@ extern int accept4(int sockFd, struct sockaddr* addr, socklen_t* addrlen, int fl
 
 // macro to calculate the length of a sockaddr_un struct for a given path length.
 // see sys/un.h#SUN_LEN, this is modified to allow nul bytes
-#define _UNIX_ADDR_LENGTH(path_len) (uintptr_t) (((struct sockaddr_un *) 0)->sun_path) + path_len
+#define _UNIX_ADDR_LENGTH(path_len) ((uintptr_t) offsetof(struct sockaddr_un, sun_path) + (uintptr_t) path_len)
 
 static int nettyNonBlockingSocket(int domain, int type, int protocol) {
 #ifdef SOCK_NONBLOCK