SECURITY: Additional CPU amplification case.

Unfortunately, commit 104870608fde3c698483fdef6b97f093fc15685d missed a case of CPU amplification via struct lists with zero-sized elements.

See advisory: https://github.com/sandstorm-io/capnproto/blob/master/security-advisories/2015-03-05-0-c++-addl-cpu-amplification.md