This corresponds to the newest reading of RFC 3629, and results
in the largest possible number of character entities by any
valid parser. This may result in a buffer which is oversized,
but never undersized.

This is after further discussion with acozzette in this PR;
https://github.com/protocolbuffers/protobuf/pull/6844

Signed-off-by: William A Rowe Jr wrowe@pivotal.io
Signed-off-by: Yechiel Kalmenson ykalmenson@pivotal.io