idec_dec: fix 0 offset of NULL pointer
idec_dec: fix 0 offset of NULL pointer

in RemapMemBuffer() and AppendToMemBuffer()

BUG=chromium:1028716,chromium:1027136

Change-Id: Ibc321d233b6207be3cb5cef4d9e8a60498e32457

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