Fix freed value in iteration exception (issue #346)

[trwyant]

This is brought on by, e.g., $ cover --coverage=default,-pod,-time Use of freed value in iteration at /Users/Shared/trw/local/perl/5.40.0/bin/cover

It requires at least two negated coverage categories to trigger the exception.

Also added recognition of negated abbreviated coverage category names (e.g. --coverage=default,-sub). These were previously ignored, though asserted abbreviations were recognized.