common/iobuf: fix memory wiping in iobuf_copy

* common/iobuf.c (iobuf_copy): Wipe used area of buffer instead of
first sizeof(char*) bytes.

(cherry picked from commit 654e353d9b20f10fa275e7ae10cc50480654f079)

• Signed-off-by: Jussi Kivilinna <jussi.kivilinna@iki.fi>