dediprog: Fix crash if usb_open() fails.

Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>
Acked-by: Stefan Tauner <stefan.tauner@student.tuwien.ac.at>

git-svn-id: svn://coreboot.org/flashrom/trunk@1705 2b7e53f0-3cfb-0310-b3e9-8179ed1497e1