Fix writing appended files to incremental backups

[vitlibar]

Changelog category:

• Bug Fix

Changelog entry:

Fix writing appended files to incremental backups. This bug could affect making backups for the tables of the Log family and also rarely for the definitions of tables.

[robot-ch-test-poll1]

This is an automated comment for commit 478140f with description of existing statuses. It's updated for the latest CI running
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[vitlibar]

Here the calculation was wrong. It started reading from the correct position but both the beginning and the end of the buffer were not adjusted correctly. As a result it correctly assigned the Content-Length for reading the appended file after the matched prefix, but passed a read buffer which couldn't read anything. As a result AWS SDK couldn't read anything from the passed read buffer and failed by timeout always.