multithread cruncher: only copy stats when picture->stats != NULL
multithread cruncher: only copy stats when picture->stats != NULL

BUG=webp:336

Change-Id: I6dfbbdcf61a6cb455f6cbf3dcd7f4c46578f42aa

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