libsnapshot: Initialize merge_op_start_ to zero.

merge_op_start_ is used to set the iterator for merge operations.
Uninitialized value can potentially lead to setting up
of bad iterator.

Bug: 233246309
Test: Full OTA
Signed-off-by: Akilesh Kailash <akailash@google.com>
Change-Id: I3cc48a66b532cfe8b2d87c8724d77ab3169a2ddb