ws: Fix invalid read when exiting
ws: Fix invalid read when exiting

This patch fixes the error below. How to reproduce: exit(ctrl+c)
with connected clients.
[2018/03/28 11:05:50:4068] NOTICE: lws_context_destroy
==3991== Invalid read of size 8
==3991==    at 0x52D0DD0: l_hashmap_foreach_remove (hashmap.c:571)
==3991==    by 0x1159B3: callback_lws_ws (proto-ws.c:452)
==3991==    by 0x5507BA6: lws_close_free_wsi_final (libwebsockets.c:581)
==3991==    by 0x5507AE2: lws_close_free_wsi (libwebsockets.c:558)
==3991==    by 0x5513F41: lws_context_destroy (context.c:838)
==3991==    by 0x115C06: ws_remove (proto-ws.c:588)
==3991==    by 0x110E65: proto_stop (proto.c:717)
==3991==    by 0x10E712: manager_stop (manager.c:333)

CESARBR/knot-service-source,CESARBR/knot-service-source,CESARBR/knot-service-source,CESARBR/knot-service-source,CESARBR/knot-service-source