Clear the destination buffer updated by the previous run in bench-memcpy.c
and test-memcpy.c to catch the error when the following implementations do
not copy anything.
[BZ #19907]
* benchtests/bench-memcpy.c (do_one_test): Clear the destination
buffer updated by the previous run.
* string/test-memcpy.c (do_one_test): Likewise.
* benchtests/bench-memmove.c (do_one_test): Add a comment.
* string/test-memmove.c (do_one_test): Likewise.