Revert "gnome-session: don't validate shell before using it"

This reverts commit db1a97c2

gnome-session/gnome-session.in

@@ -2,7 +2,10 @@
 
 if [ "x$XDG_SESSION_TYPE" = "xwayland" ] &&
    [ "x$XDG_SESSION_CLASS" != "xgreeter" ] &&
-   [  -n "$SHELL" ]; then
+   [  -n "$SHELL" ] &&
+   grep -q "$SHELL" /etc/shells &&
+   ! (echo "$SHELL" | grep -q "false") &&
+   ! (echo "$SHELL" | grep -q "nologin"); then
   if [ "$1" != '-l' ]; then
     exec bash -c "exec -l '$SHELL' -c '$0 -l $*'"
   else