libstdc++ Fix undefined behaviour in testsuite

Fix some test bugs found by ubsan.

libstdc++-v3/ChangeLog:

	* testsuite/20_util/from_chars/3.cc: Use unsigned type to avoid
	overflow.
	* testsuite/24_iterators/reverse_iterator/2.cc: Do not add
	non-zero value to null pointer.
	* testsuite/25_algorithms/copy_backward/move_iterators/69478.cc:
	Use past-the-end iterator for result.
	* testsuite/25_algorithms/move_backward/69478.cc: Likewise.
	* testsuite/25_algorithms/move_backward/93872.cc: Likewise.