We crash when parsing

  [[a::

because we see a CPP_SCOPE and then we're trying to consume a CPP_EOF
token.  So peek before consuming it.

	PR c++/93684 - ICE-on-invalid with broken attribute.
	* parser.c (cp_parser_std_attribute): Peek a token first before
	consuming it.

	* g++.dg/parse/attr4.C: New test.