Fixed: make sure there is a valid handle before do a reset.
Fixed: make sure there is a valid handle before do a reset.

If reset method is called without a valid device handle, we will
probably have a crash. So, call managed_open before calling reset.

AlanJAS/pyusb,h3nj3/pyusb,karlp/pyusb,rabryan/pyusb,h3nj3/pyusb,makerbot/pyusb,AlanJAS/pyusb,karlp/pyusb,makerbot/pyusb,kopchik/pyusb,rabryan/pyusb,swinman/pyusb,kopchik/pyusb,swinman/pyusb