Don't start sql connection pools for unknown hosts

It's possible to trigger that by passing wrong domain to some ctl commands,
and if you don't have default credentials for sql connections, you get
repeating errors when this pools tries to reconnect from error.

src/ejabberd_sql_sup.erl

@@ -37,6 +37,11 @@ start(Host) ->
     case is_started(Host) of
 	true -> ok;
 	false ->
+	    case lists:member(Host, ejabberd_option:hosts()) of
+		false ->
+		    ?WARNING_MSG("Rejecting start of sql worker for unknown host: ~ts", [Host]),
+		    {error, invalid_host};
+		true ->
 		    App = case ejabberd_option:sql_type(Host) of
 			      mysql -> p1_mysql;
 			      pgsql -> p1_pgsql;
@@ -62,6 +67,7 @@ start(Host) ->
 			    ?ERROR_MSG("Failed to start ~ts: ~p", [?MODULE, Why]),
 			    Err
 		    end
+	    end
     end.
 
 stop(Host) ->