Wait for all threads to be done in DecodeRemaining.
Wait for all threads to be done in DecodeRemaining.

This could lead to a race condition.

BUG=oss-fuzz:9417

Change-Id: Ifef4001115888541cb76095e1c0226487d497b78

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