fr-command-rar: check for NULL in name_field

When I tried to open rar archive from floppy disk (floppy corrupted), I
got crash.
149		if (*name_field == '/') {
(gdb) p name_field
$3 = 0x0
Sure, it can't be dereferenced.

Reference: https://bugzilla.redhat.com/show_bug.cgi?id=1135409
Reference: https://bugzilla.gnome.org/show_bug.cgi?id=735657
Signed-off-by: Igor Gnatenko <i.gnatenko.brain@gmail.com>
rar: do not return NULL