re PR tree-optimization/71272 (internal compiler error: in operator[], through tree-ssa-threadupdate.c:1981)

	PR tree-optimization/71272
	* tree-ssa-threadbackward.c (convert_and_register_jump_thread_path):
	Update comments.  Add test for empty path.

	PR tree-optimization/71272
	* gcc.c-torture/compile/pr71272.c: new test.

From-SVN: r236755