chfn: don't append extra tailing commas

chfn: don't append extra tailing commas

 # grep kzak /etc/passwd
 kzak:x:1000:1000::/home/kzak:/bin/bash

 # chfn kzak
 ...
 grep kzak /etc/passwd

old version:
 kzak:x:1000:1000:Karel Zak,,,,:/home/kzak:/bin/bash

fixed version:
 kzak:x:1000:1000:Karel Zak:/home/kzak:/bin/bash

Reported-by: Filip Dvorak <fdvorak@redhat.com>
References: f723cbf544a7eac2927634f2cb6d802437a2d519
Signed-off-by: Karel Zak <kzak@redhat.com>