Set closecb to NULL on update requests' connections at deinit

Otherwise the closecb is called when the connection is closed/freed during
deinit, and this happens in the HTTP cleanup after the DAAP and DACP cleanups
have run, leading to spurious "struct update_request not found" messages.

src/httpd_daap.c

@@ -2870,6 +2870,7 @@ daap_deinit(void)
     {
       update_requests = ur->next;
 
+      evhttp_connection_set_closecb(ur->req->evcon, NULL, NULL);
       free(ur);
     }
 }

src/httpd_dacp.c

@@ -653,6 +653,7 @@ dacp_deinit(void)
     {
       update_requests = ur->next;
 
+      evhttp_connection_set_closecb(ur->req->evcon, NULL, NULL);
       free(ur);
     }
 }