Speed up hash chain initialization using memset.
Speed up hash chain initialization using memset.

That gains 1% on lossy compression.

Change-Id: Ib9aa210194ed2f17eaff85b499b55cc4eb99ff11

PKRoma/libwebp,webmproject/libwebp,webmproject/libwebp,ttyangf/libwebp,ttyangf/libwebp,PKRoma/libwebp,PKRoma/libwebp,PKRoma/libwebp,ttyangf/libwebp,webmproject/libwebp,ttyangf/libwebp,PKRoma/libwebp,ttyangf/libwebp,webmproject/libwebp,PKRoma/libwebp,webmproject/libwebp