libsnapshot: Initialize merge_op_start_ to zero. am: d36a68183c am: 1ba9714583

Original change: https://googleplex-android-review.googlesource.com/c/platform/system/core/+/18512862

Change-Id: Ie5600149c4af84e480a683ae4e245102c47fc917
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>