Fix terms agg error calculation if shared results are reduced incrementally.

During #23253 an issue raised with the terms aggs error calculation. Details are attached to the test case [here](https://github.com/elastic/elasticsearch/blob/master/core/src/test/java/org/elasticsearch/search/aggregations/bucket/TermsDocCountErrorIT.java#L72)

I need help with this from either @jpountz and / or @colings86 