entry completion: add checks that entry is set already

If any of the APIs that assumes that the entry is set already is used
before having one already set, things break pretty badly.

Fixes a downstream issue reported at gtk-rs/gtk4-rs#873

gtk/gtkentrycompletion.c

@@ -1211,6 +1211,8 @@ gtk_entry_completion_match_selected (GtkEntryCompletion *completion,
                                      GtkTreeModel       *model,
                                      GtkTreeIter        *iter)
 {
+  g_assert (completion->entry != NULL);
+
   char *str = NULL;
 
   gtk_tree_model_get (model, iter, completion->text_column, &str, -1);
@@ -1229,6 +1231,8 @@ gtk_entry_completion_cursor_on_match (GtkEntryCompletion *completion,
                                       GtkTreeModel       *model,
                                       GtkTreeIter        *iter)
 {
+  g_assert (completion->entry != NULL);
+
   gtk_entry_completion_insert_completion (completion, model, iter);
 
   return TRUE;
@@ -1317,6 +1321,8 @@ static gboolean
 gtk_entry_completion_real_insert_prefix (GtkEntryCompletion *completion,
                                          const char         *prefix)
 {
+  g_assert (completion->entry != NULL);
+
   if (prefix)
     {
       int key_len;
@@ -1417,6 +1423,8 @@ gtk_entry_completion_insert_completion (GtkEntryCompletion *completion,
 void
 gtk_entry_completion_insert_prefix (GtkEntryCompletion *completion)
 {
+  g_return_if_fail (completion->entry != NULL);
+
   gboolean done;
   char *prefix;
   GtkText *text = gtk_entry_get_text_widget (GTK_ENTRY (completion->entry));