Add missing null pointer check in gnc_account_renumber_update_examples()

gtk_entry_get_text() should never return null but "prefix" is checked for
null on the first string and then dereferenced unconditionally on the
second string. Change this to be consistent and check twice.

gnucash/gnome-utils/dialog-account.c

@@ -2232,7 +2232,7 @@ gnc_account_renumber_update_examples (RenumberDialog *data)
     gtk_label_set_text (GTK_LABEL(data->example1), str);
     g_free (str);
 
-    if (strlen (prefix))
+    if (prefix && *prefix)
         str = g_strdup_printf ("%s-%0*d", prefix, num_digits,
                                interval * data->num_children);
     else