Fix possible AttributeError when account-api is called over unix socket (#16404)

Fixes #16396

changelog.d/16404.bugfix

@@ -0,0 +1 @@
+Fixes possbile `AttributeError` when `_matrix/client/v3/account/whoami` is called over a unix socket. Contributed by @Sir-Photch.

synapse/api/auth/internal.py

@@ -115,7 +115,7 @@ class InternalAuth(BaseAuth):
         Once get_user_by_req has set up the opentracing span, this does the actual work.
         """
         try:
-            ip_addr = request.getClientAddress().host
+            ip_addr = request.get_client_ip_if_available()
             user_agent = get_request_user_agent(request)
 
             access_token = self.get_access_token_from_request(request)