BUG/MINOR: cli: Don't warn about a too big command for incomplete commands

BUG/MINOR: cli: Don't warn about a too big command for incomplete commands

When a command is too big to fit in a buffer, a error is returned before
closing. However, the error is also returned if the command is small enough
but incomplete. It happens on abort. In this case, the error must not be
reported. The regression was introduced when a dedicated sn_buf callbac
function was added.

To fix the issue, both cases are now handled separately.

No backport needed.