Check return value of TableBuilder::Add() in compaction routine.

[evanstade]

This is a prospective fix for the crash described in crbug.com/326566884. Inspection of the crash dump suggests that Add() is failing, but the compaction routine is continuing to run. While it's not entirely clear how precisely this leads to the observed failure mode, it seems safer to check the status of the TableBuilder and abort early as needed.