Fix for issue #645

This is the minimal fix for this issue. Before this fix, there were only two places where a corrupted file could show up: 1. From a curl download that fails without an error code 2. md380-fw generating a corrupted file The other recipes in the firmware directory all check the size of the target and delete it and fail if there is a mismatch. This minimal fix fills gap number 1 from above with a simple check for greater than zero bytes.