Only set FLOATING_AUTO_ON when floating_enable succeeds

Fixes #4039

Crash with docking clients where the floating field is set even though
floating_enable refuses to make them floating.

See issue for example with logs.

src/floating.c

@@ -221,22 +221,22 @@ void floating_check_size(Con *floating_con, bool prefer_height) {
     }
 }
 
-void floating_enable(Con *con, bool automatic) {
+bool floating_enable(Con *con, bool automatic) {
     bool set_focus = (con == focused);
 
     if (con_is_docked(con)) {
         LOG("Container is a dock window, not enabling floating mode.\n");
-        return;
+        return false;
     }
 
     if (con_is_floating(con)) {
         LOG("Container is already in floating mode, not doing anything.\n");
-        return;
+        return false;
     }
 
     if (con->type == CT_WORKSPACE) {
         LOG("Container is a workspace, not enabling floating mode.\n");
-        return;
+        return false;
     }
 
     Con *focus_head_placeholder = NULL;
@@ -419,6 +419,7 @@ void floating_enable(Con *con, bool automatic) {
 
     floating_set_hint_atom(nc, true);
     ipc_send_window_event("floating", con);
+    return true;
 }
 
 void floating_disable(Con *con) {

src/handlers.c

@@ -841,12 +841,13 @@ static void handle_client_message(xcb_client_message_event_t *event) {
              * let's float it and make it sticky. */
             DLOG("The window was requested to be visible on all workspaces, making it sticky and floating.\n");
 
-            floating_enable(con, false);
-            con->floating = FLOATING_AUTO_ON;
+            if (floating_enable(con, false)) {
+                con->floating = FLOATING_AUTO_ON;
 
-            con->sticky = true;
-            ewmh_update_sticky(con->window->id, true);
-            output_push_sticky_windows(focused);
+                con->sticky = true;
+                ewmh_update_sticky(con->window->id, true);
+                output_push_sticky_windows(focused);
+            }
         } else {
             Con *ws = ewmh_get_workspace_by_index(index);
             if (ws == NULL) {

src/manage.c

@@ -537,8 +537,9 @@ void manage_window(xcb_window_t window, xcb_get_window_attributes_cookie_t cooki
          * was not specified */
         bool automatic_border = (motif_border_style == BS_NORMAL);
 
-        floating_enable(nc, automatic_border);
-        nc->floating = FLOATING_AUTO_ON;
+        if (floating_enable(nc, automatic_border)) {
+            nc->floating = FLOATING_AUTO_ON;
+        }
     }
 
     /* explicitly set the border width to the default */