Fixed bug #478 to correctly reallocate buffer

Podhorszki, Norbertrequested to merge
github/fork/williamfgc/write_bug_478 into master

Created by: williamfgc

Caused when reallocating metadata in data. Capacity was sufficient, while size was not. Enforcing capacity change with reserve (expensive) before a resize (cheap). Also, skipping PerformPuts if there is zero deferred variables. this was resizing the buffer before EndStep.

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