get_output_for_con: Assert result != NULL

- The result from con_get_output was always not NULL because
con_get_output asserts so
- get_output_by_name should always be able to get an output from the
corresponding container
- workspace_move_to_output doesn't return bool anymore since it can't
fail

src/con.c

@@ -1400,8 +1400,6 @@ void con_move_to_output(Con *con, Output *output, bool fix_coordinates) {
  */
 bool con_move_to_output_name(Con *con, const char *name, bool fix_coordinates) {
     Output *current_output = get_output_for_con(con);
-    assert(current_output != NULL);
-
     Output *output = get_output_from_string(current_output, name);
     if (output == NULL) {
         ELOG("Could not find output \"%s\"\n", name);