basic: make check-pointer-deref clean

Add the needed assert changes to make the code clean
for the new check-pointer-deref script.

Note that for `_cmp()` functions its a bit of a question
if we want `assert()` or `POINTER_MAY_BE_NULL()` here. The
former is more correct but it adds a (small) runtime hit. If
we use POINTER_MAY_BE_NULL() it is compiled out but strictly
speaking it is wrong (so at least we would need a comment).

src/basic/socket-util.c

@@ -360,6 +360,7 @@ int sockaddr_port(const struct sockaddr *_sa, unsigned *ret_port) {
         /* Note, this returns the port as 'unsigned' rather than 'uint16_t', as AF_VSOCK knows larger ports */
 
         assert(sa);
+        assert(ret_port);
 
         switch (sa->sa.sa_family) {
 
@@ -824,6 +825,8 @@ bool ifname_valid_full(const char *p, IfnameValidFlags flags) {
 
 bool address_label_valid(const char *p) {
 
+        POINTER_MAY_BE_NULL(p);
+
         if (isempty(p))
                 return false;
 
@@ -1556,6 +1559,8 @@ int socket_set_option(int fd, int af, int opt_ipv4, int opt_ipv6, int val) {
 int socket_get_mtu(int fd, int af, size_t *ret) {
         int mtu, r;
 
+        assert(ret);
+
         if (af == AF_UNSPEC) {
                 af = socket_get_family(fd);
                 if (af < 0)