res_http_websocket: When shutting down a session don't close closed socket

Due to the use of ast_websocket_close in session termination it is
possible for the underlying socket to already be closed when the
session is terminated. This occurs when the close frame is attempted
to be written out but fails.

Change-Id: I7572583529a42a7dc911ea77a974d8307d5c0c8b