Corrected error of fw-admin -c

-Before introduced fw-admin -c without file

fw-admin -c
I: Checking variables /etc/fw-admin.d/rules/
grep: /etc/fw-admin.d/rules/: Es un directorio
grep: /etc/fw-admin.d/rules/: Es un directorio
grep: /etc/fw-admin.d/rules/: Es un directorio
I: Checking iptables syntax /etc/fw-admin.d/rules/
cat: /etc/fw-admin.d/rules/: Es un directorio
I: Checking ip6tables syntax /etc/fw-admin.d/rules/
cat: /etc/fw-admin.d/rules/: Es un directorio

-Now introduce fw-admin -c without file

E: Please specify a file to check

Works as it should

Signed-off-by: Christian Mora Moreno <christian.mora@cica.es>