Fix async insert with deduplication for ReplicatedMergeTree using merging algorithms

[antonio2368]

Changelog category (leave one):

• Bug Fix (user-visible misbehavior in an official stable release)

Changelog entry (a user-readable short description of the changes that goes to CHANGELOG.md):

Fix crash when async inserts with deduplication are used for ReplicatedMergeTree tables using a nondefault merging algorithm.

cc @hanfei1991 @alesapin

Documentation entry for user-facing changes

• Documentation is written (mandatory for new features)

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