st/entry: Fix leak when copying or cutting text using shortcuts

clutter_text_get_selection() creates a copy of the selected text which
gets passed to st_clipboard_set_text() which creates its own copy. The
copy returned by clutter_text_get_selection() however never got free'd.

https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1306


(cherry picked from commit fdfcacf1db)

src/st/st-entry.c

@@ -679,6 +679,8 @@ st_entry_key_press_event (ClutterActor    *actor,
                                ST_CLIPBOARD_TYPE_CLIPBOARD,
                                text);
 
+      g_free (text);
+
       return TRUE;
     }
 
@@ -705,6 +707,8 @@ st_entry_key_press_event (ClutterActor    *actor,
           clutter_text_delete_selection ((ClutterText *) priv->entry);
         }
 
+      g_free (text);
+
       return TRUE;
     }