gnrc_netif2: make gnrc_netif2_create idempotent and crash on error

sys/include/net/gnrc/netif2.h

@@ -215,8 +215,11 @@ struct gnrc_netif2_ops {
  * @note If @ref DEVELHELP is defined netif_params_t::name is used as the
  *       name of the network interface's thread.
  *
+ * @attention   Fails and crashes (assertion error with @ref DEVELHELP or
+ *              segmentation fault without) if `GNRC_NETIF_NUMOF` is lower than
+ *              the number of calls to this function.
+ *
  * @return  The network interface on success.
- * @return  NULL, on error.
  */
 gnrc_netif2_t *gnrc_netif2_create(char *stack, int stacksize, char priority,
                                   const char *name, netdev_t *dev,

sys/include/net/gnrc/netif2/ethernet.h

@@ -35,8 +35,11 @@ extern "C" {
  *
  * @see @ref gnrc_netif2_create()
  *
+ * @attention   Fails and crashes (assertion error with @ref DEVELHELP or
+ *              segmentation fault without) if `GNRC_NETIF_NUMOF` is lower than
+ *              the number of calls to this function.
+ *
  * @return  The network interface on success.
- * @return  NULL, on error.
  */
 gnrc_netif2_t *gnrc_netif2_ethernet_create(char *stack, int stacksize, char priority,
                                            char *name, netdev_t *dev);

sys/net/gnrc/netif2/gnrc_netif2.c

@@ -51,16 +51,14 @@ gnrc_netif2_t *gnrc_netif2_create(char *stack, int stacksize, char priority,
     int res;
 
     for (int i = 0; i < GNRC_NETIF_NUMOF; i++) {
-        if (_netifs[i].ops == NULL) {
+        if (_netifs[i].dev == netdev) {
+            return &_netifs[i];
+        }
+        if ((netif == NULL) && (_netifs[i].ops == NULL)) {
             netif = &_netifs[i];
-            break;
         }
     }
-    if (netif == NULL) {
-        LOG_ERROR("gnrc_netif2: can not allocate network interface.\n"
-                  "Set GNRC_NETIF_NUMOF to a higher value\n");
-        return NULL;
-    }
+    assert(netif != NULL);
     rmutex_init(&netif->mutex);
     netif->ops = ops;
     assert(netif->dev == NULL);