policycoreutils/chcat: Add a fallback in case os.getlogin() returns nothing

Some teminal emulators (like the latest version of gnome-terminal) are
not setting entries in the utmp file, this leads getlogin() to return an
empty string.

Fallback to the name of the user running the chcat process.

policycoreutils/scripts/chcat

@@ -372,7 +372,10 @@ def listcats():
 
 def listusercats(users):
     if len(users) == 0:
-        users.append(os.getlogin())
+        try:
+            users.append(os.getlogin())
+        except:
+            users.append(pwd.getpwuid(os.getuid()).pw_name)
 
     verify_users(users)
     for u in users: