Set memory buffer size to 0 for OutputChecker

Summary:
Travis builds reported MemoryError using the previous buffer size of 2^16 bytes.
More profiling is needed to determine the exact cause of the Travis problem.

Links: T53

Test Plan: qa/pull-tester/rpc-tests.py -extended

Reviewers: #bitcoin_abc, deadalnix, sickpig, kyuupichan, awemany

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D257