remove trans_info parameter from dissect_usb_endpoint_descriptor()
(remove it in the call from mausb as well)

add NULL checks for usb_conv_info, usb_trans_info

Change-Id: I95d52b85e3fa0d42b6a20047053259aaca8754fe
Reviewed-on: https://code.wireshark.org/review/3784
Reviewed-by: Martin Kaiser <wireshark@kaiser.cx>