test_hexdump: use memcpy instead of strncpy

commit b1286ed7158e9b62787508066283ab0b8850b518 upstream.

New versions of gcc reasonably warn about the odd pattern of

	strncpy(p, q, strlen(q));

which really doesn't make sense: the strncpy() ends up being just a slow
and odd way to write memcpy() in this case.

Apparently there was a patch for this floating around earlier, but it
got lost.

Acked-again-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>