[BP 7.1.4] - XDCR panic when filtering

This happen when

1. Filter uses document key such as META().id = "docName"
2. The document being filtered has uncommitted transaction XATTRs

In line https://github.com/couchbase/goxdcr/blob/9c6fce2e565f87fdb7abd6503c32f2a40593d928/base/filter/filterUtils.go#L90

Filter overwrites the previous body with a smaller one, the endBodyPos was kept at the old value, causing panic at https://github.com/couchbase/goxdcr/blob/9c6fce2e565f87fdb7abd6503c32f2a40593d928/base/filter/filterUtils.go#L117