Ensure any badly formatted index docs are filtered
Ensure any badly formatted index docs are filtered

- as index structuring happens async, there can be some cases where queries will match stored docs but between matching and fetching, the doc becomes unavailable (say if a doc gets deleted but interupted)
- in any case, undefined results should be filtered out in case they do exist, as they will not be able to be mapped back to pouch (causing TypeErrors when access happens)
- addresses #94 (part 2)

WorldBrain/WebMemex,WorldBrain/WebMemex