Fixed SEGV in sub-message getters for well-known types when message is unset.

[haberman]

The well-known types generate C code into wkt.inc, and this C code was not testing isset($msg->submsg_field) like the generated code does:

// PHP generated getter: checks isset().
public function getOptions()
{
    return isset($this->options) ? $this->options : null;
}

// C generated getter, does not check upb_msg_has()
static PHP_METHOD(google_protobuf_Value, getListValue) {
  Message* intern = (Message*)Z_OBJ_P(getThis());
  const upb_fielddef *f = upb_msgdef_ntofz(intern->desc->msgdef,
                                           "list_value");
  zval ret;
  Message_get(intern, f, &ret);
  RETURN_COPY_VALUE(&ret);
}

This led to an error in well-known type getters like getListValue() above where we would try to get a sub-message field from upb when upb_msg_has(msg, field) == false, which is an error according to upb.

There are two possible fixes for this bug. A guiding principle is that we want the generated C code in wkt.inc to have the same behavior as PHP generated code. Following this principle, the two possible fixes are:

1. Change the code generator for wkt.inc to check upb_msg_has(f) before calling Message_get(). This would match the isset() check that the The PHP generated code does, and we would leave the PHP code unchanged.

2. Change Message_get() to itself perform the upb_msg_has(f) check for sub-message fields. This means that generated code would no longer need to perform an isset() check, so we would want to remove this check from the PHP generated code also to avoid a redundant check.

Both of these are reasonable fixes, and it is not immediately obvious which is better. (1) has the benefit of resolving this case when we are in more specialized code (a getter function that already knows this is a sub-message field), and therefore avoids performing the check later in more generic code that would have to test the type again. On the other hand, the isset() check is not needed for the pure PHP implementation, as an unset PHP variable will return null anyway. And for the C extension, we'd rather check upb_msg_has() at the C level instead of PHP for performance reasons.

So this change implements (2). The generated code in wkt.inc remains unchanged, and the PHP generated code for sub-message fields is changed to remove the isset() check.

Fixes: #8659