endpoint.c: Plug a memory leak in ast_endpoint_shutdown().

endpoint.c: Plug a memory leak in ast_endpoint_shutdown().

Commit 26795be introduced a memory leak of ast_endpoint when
ast_endpoint_shutdown() was called. The leak occurs only if a configuration
change removes an endpoint and isn't related to call volume or the length of
time asterisk has been running.  An ao2_ref(-1) has been added to
ast_endpoint_shutdown() to plug the leak.

Resolves: #1635