Start ezlib only if required, as it's optional (#1006)

src/ejabberd_app.erl

@@ -256,7 +256,6 @@ start_apps() ->
     ejabberd:start_app(fast_tls),
     ejabberd:start_app(fast_xml),
     ejabberd:start_app(stringprep),
-    ejabberd:start_app(ezlib),
     ejabberd:start_app(cache_tab).
 
 opt_type(net_ticktime) ->

src/ejabberd_receiver.erl

@@ -141,6 +141,7 @@ handle_call({starttls, TLSSocket}, _From, State) ->
 handle_call({compress, Data}, _From,
 	    #state{socket = Socket, sock_mod = SockMod} =
 		State) ->
+    ejabberd:start_app(ezlib),
     {ok, ZlibSocket} = ezlib:enable_zlib(SockMod,
 						 Socket),
     if Data /= undefined -> do_send(State, Data);