makechrootpkg: Stat chrootdir type after existancy

Move detection of chrootdir type after have check if the directory exists.

This avoid the following messages when -r is not given
stat: cannot read file system information for '': No such file or directory

Signed-off-by: Sébastien Luttringer <seblu@seblu.net>
Signed-off-by: Pierre Schmitz <pierre@archlinux.de>