Issue 403: Buffer underflow parsing 'ar' header

Issue 403: Buffer underflow parsing 'ar' header

While pruning trailing text from ar filenames, we did not
check for an empty filename.  This results in reading the byte
before the filename on the stack.

While here, change a number of ar format issues from WARN to FATAL.
It's better to abort on a damaged file than risk reading garbage.
No doubt, this will require additional tuning in the future.