su: make code more robust [coverity scan]

... and more readable for static analyzers

Signed-off-by: Karel Zak <kzak@redhat.com>

login-utils/su-common.c

@@ -387,6 +387,9 @@ authenticate (const struct passwd *pw)
   case RUNUSER_MODE:
     srvname = simulate_login ? PAM_SRVNAME_RUNUSER_L : PAM_SRVNAME_RUNUSER;
     break;
+  default:
+    abort();
+    break;
   }
 
   retval = pam_start (srvname, pw->pw_name, &conv, &pamh);