gccrs: Fix segv in unsafe chcker

gccrs: Fix segv in unsafe chcker

Trait constants were missing type resolution step, this adds that
as if it was a normal constant. The unsafe checker was missing a
null check.

Fixes Rust-GCC#3612

gcc/rust/ChangeLog:

	* checks/errors/rust-unsafe-checker.cc (UnsafeChecker::visit): add null check
	* hir/tree/rust-hir-item.h: add has_type helper
	* typecheck/rust-hir-trait-resolve.cc (TraitItemReference::resolve_item):
	add missing type checking

gcc/testsuite/ChangeLog:

	* rust/compile/issue-3612.rs: New test.

Signed-off-by: Philip Herron <herron.philip@googlemail.com>