Aireplay-ng: Use variable name (instead of hardcoded value) when checking validity of the length of the data to read.

git-svn-id: http://svn.aircrack-ng.org/trunk@2420 28c6078b-6c39-48e3-add9-af49d547ecab
Aireplay-ng test: Better handling of invalid length (or empty stuff).

git-svn-id: http://svn.aircrack-ng.org/trunk@2421 28c6078b-6c39-48e3-add9-af49d547ecab