Fix LOGICAL_ERROR in getMaxSourcePartsSizeForMerge with new config.xml parameters

Fix LOGICAL_ERROR in getMaxSourcePartsSizeForMerge during merges (in case of non standard, greater, values of background_pool_size/background_merges_mutations_concurrency_ratio has been specified in config.xml (new way) not in users.xml (deprecated way)). #37413 (Azat Khuzhin).