all: use nm_memdup() instead of g_memdup()

g_memdup()'s size argument is a guint. There was CVE-2021-27219
about an integer overflow, which results in a buffer overflow.
In response to that, g_memdup2() was introduced in 2.68.

We can't use g_memdup2(), because our currently required glib
version is still 2.40.

There was no bug at those two places where g_memdup() was used.
It's just that g_memdup() is a code smell. Prevent any questions that
a reader of the code might have regarding the correctness of g_memdup()
(w.r.t. integer/buffer overflow), by not using it.

Instead use our internal nm_memdup() variant, which exactly exists for
this reason.

See-also: https://gitlab.gnome.org/GNOME/glib/-/issues/2319

src/libnm-client-impl/nm-secret-agent-old.c

@@ -818,7 +818,7 @@ nm_secret_agent_old_register_async(NMSecretAgentOld   *self,
             cancelled_id =
                 g_cancellable_connect(cancellable, G_CALLBACK(_register_cancelled_cb), task, NULL);
             if (cancelled_id != 0) {
-                g_task_set_task_data(task, g_memdup(&cancelled_id, sizeof(cancelled_id)), g_free);
+                g_task_set_task_data(task, nm_memdup(&cancelled_id, sizeof(cancelled_id)), g_free);
             }
         }
     }

src/libnm-glib-aux/nm-test-utils.h

@@ -1939,7 +1939,7 @@ nmtst_logging_disable(gboolean always)
         return NULL;
     }
 
-    p = g_memdup(_nm_logging_enabled_state, sizeof(_nm_logging_enabled_state));
+    p = nm_memdup(_nm_logging_enabled_state, sizeof(_nm_logging_enabled_state));
     memset(_nm_logging_enabled_state, 0, sizeof(_nm_logging_enabled_state));
     return p;
 }