clear the temporary components of usb_conv_info_t before we dissect a
new packet

this fixes the fuzz failures in
https://bugs.wireshark.org/bugzilla/show_bug.cgi?id=10386

Change-Id: I8c0ba8c8da9af8d7a3ef9cdd7781769b9966f1a2
Reviewed-on: https://code.wireshark.org/review/3727
Reviewed-by: Martin Kaiser <wireshark@kaiser.cx>
Tested-by: Martin Kaiser <wireshark@kaiser.cx>