fix FulgoraGraphComputer readBatchSize number overflow fault

[amcp]

@mbroecheler In FulgoraGraphComputer,
readBatchSize is fed into StandardScanner.Builder.setWorkBlockSize which throws for negative numbers. If readBatchSize is negative because of 10 * storage.buffer-size > Integer.MAX_VALUE, then the fault is triggered.
Instead, check writeBatchSize=storage.buffer-size in FulgoraGraphComputer constructor and throw if too large.