BUG/MEDIUM: cli: Properly parse empty lines and avoid crashed

BUG/MEDIUM: cli: Properly parse empty lines and avoid crashed

Empty lines was not properly parsed and could lead to crashes because the
last argument was parsed outside of the cmdline buffer. Indeed, the last
argument is parsed to look for an eventual payload pattern. It is started
one character after the newline at the end of the command line. But it is
only valid for an non-empty command line.

So, now, this case is properly detected when we leave if an empty line is
detected.

This patch must be backported to 3.2.