Fix a buffer overflow.

This is only the minimum workaround to prevent buffer overflow:
Stop iterating once the (fixed!) size of the output buffers is
reached. In response to
https://www.talosintelligence.com/vulnerability_reports/TALOS-2017-0317

However, this code is a huge mess anyway and is in no way
anything like up-to-date C++ code. Please, anyone, replace it
with something more modern. Thanks.