libsnapshot: Initialize merge_op_start_ to zero. am: d36a68183c

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

Change-Id: I8db49bc8d5f2b920f7817c0f3e61456bd8c30a07
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>