No check was performed and the value was assumed non null.
Path conversion returned an empty path for "Self" due to a hack in
generics Self injection that prevented any "Self!" macro from beeing
recognized correctly.

gcc/rust/ChangeLog:

	* parse/rust-parse-impl.h (Parser::parse_stmt_or_expr): Add null check
	on parse_macro_invocation_partial call.
	* ast/rust-path.cc (Path::convert_to_simple_path): Do not exclude
	capitalized "Self".

gcc/testsuite/ChangeLog:

	* rust/compile/issue-3974.rs: New test.

Signed-off-by: Pierre-Emmanuel Patry <[email protected]>