gnrc_ipv6_nib/nc: remove from _next_removable on remove

The `_next_removable` list manages the cache-out of the neighbor cache.
However, when a neighbor cache entry is removed, it is not removed
from that list, which may lead to a segmentation fault when that list is
accessed, since the whole entry (including its list pointer) is zeroed
after removal.

With this change the entry is removed from that list accordingly before
the zeroing happens.

sys/net/gnrc/network_layer/ipv6/nib/_nib-internal.c

@@ -272,6 +272,8 @@ void _nib_nc_remove(_nib_onl_entry_t *node)
         entry->pkt = NULL;
     }
 #endif  /* GNRC_IPV6_NIB_CONF_QUEUE_PKT */
+    /* remove from cache-out procedure */
+    clist_remove(&_next_removable, (clist_node_t *)node);
     _nib_onl_clear(node);
 }