DecodeImageData(): change the incorrect assert
DecodeImageData(): change the incorrect assert

this function can be called not to decode pixels, but simply
to finish processing (through process_func()) the already decoded
pixels.

Change-Id: I80485e92e3c47f0aa3389476dcb82745a243fc4a

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