update_engine: remove size_t in FEC write path am: e0a9543b76

Original change: https://android-review.googlesource.com/c/platform/system/update_engine/+/3248266

Change-Id: If8ecfdf1fa08faf2d25abd417f26fb614fc2c06a
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>