fix a crash in the endpoint dialogue

compile Wireshark with GeoIP enabled
load a USB capture
open Statistics / Endpoints
   the tree is empty, USB is not amongst the selected Enpoint Types
select USB in the Endpoint Types list
-> crash

to fix this, add a NULL check for cur_tree to EndpointDialog::tabChanged()

Change-Id: I43d963ae5cd451120ac354e89e37ccbbbc2c5910
Reviewed-on: https://code.wireshark.org/review/5192
Reviewed-by: Martin Kaiser <wireshark@kaiser.cx>
Tested-by: Martin Kaiser <wireshark@kaiser.cx>