util/cbfstool: Make cbfs_image_delete() NULL-tolerant.

This fixes a double free crash that occurs when a call to
cbfs_image_from_file() fails in cbfs_extract() and falls though to
cbfs_image_delete() with a NULL-pointer.

To reproduce the crash pass the following arguments where the files
passed, in fact, do not exist. As follows:
./cbfstool build/coreboot.rom extract -n config -f /tmp/config.txt

Change-Id: I2213ff175d0703705a0ec10271b30bb26b6f8d0a
Signed-off-by: Edward O'Callaghan <eocallaghan@alterapraxis.com>
Reviewed-on: http://review.coreboot.org/5353
Tested-by: build bot (Jenkins)
Reviewed-by: Alexandru Gagniuc <mr.nuke.me@gmail.com>