libwebsockets

vhost destruction: dont allow all wsi closures to kill vh we are already in process of destroying

Saw this on travis selftests during context destroy

==18895== Invalid read of size 8
==18895==    at 0x415909: __lws_vhost_destroy2 (vhost.c:1063)
==18895==    by 0x40E65B: lws_context_destroy2 (context.c:929)
==18895==    by 0x40EBE5: lws_context_destroy (context.c:1128)
==18895==    by 0x40CC41: main (minimal-http-client-post.c:267)
==18895==  Address 0x6168688 is 728 bytes inside a block of size 792 free'd
==18895==    at 0x4C2BDEC: free (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==18895==    by 0x45B29E: _realloc (alloc.c:120)
==18895==    by 0x45B2D6: lws_realloc (alloc.c:130)
==18895==    by 0x415ED7: __lws_vhost_destroy2 (vhost.c:1204)
==18895==    by 0x419164: lws_vhost_unbind_wsi (wsi.c:82)
==18895==    by 0x41236B: __lws_free_wsi (close.c:154)
==18895==    by 0x4134CF: __lws_close_free_wsi_final (close.c:650)
==18895==    by 0x4133BA: __lws_close_free_wsi (close.c:610)
==18895==    by 0x413528: lws_close_free_wsi (close.c:660)
==18895==    by 0x4158C7: __lws_vhost_destroy2 (vhost.c:1053)
==18895==    by 0x40E65B: lws_context_destroy2 (context.c:929)
==18895==    by 0x40EBE5: lws_context_destroy (context.c:1128)

Removing the last wsi from the vhost we started to destroy finalized the
vhost destruction, which is aimed at libuv async close cleanup.  But if
we already entered __lws_vhost_destroy2, we will definitely destroy the vhost
ourselves at the end of that function already.  So defeat the wsi close
triggering it.