Objective-C++ : Avoid ICE on invalid with empty attributes.

Objective-C++ : Avoid ICE on invalid with empty attributes.

Empty prefix attributes like:

__attribute__ (())
@interface MyClass
@end

cause an ICE at present, check for that case and skip them.

gcc/cp/ChangeLog:

	* parser.c (cp_parser_objc_valid_prefix_attributes): Check
	for empty attributes.