gccrs: Fix SEGV when type path resolver fails outright

gccrs: Fix SEGV when type path resolver fails outright

When we resolve paths we resolve to Types first we walk each segment to
the last module which has no type but then in the event that the child
of a module is not found we have a null root_tyty which needs to be caught
and turned into an ErrorType node.

Fixes Rust-GCC#3613

gcc/rust/ChangeLog:

	* typecheck/rust-hir-type-check-type.cc (TypeCheckType::resolve_root_path):
	catch nullptr root_tyty

gcc/testsuite/ChangeLog:

	* rust/compile/issue-3613.rs: New test.

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