[reSIProcate] assert in Connection::ensureWritable()

[Daniel Pocock]

Here is one assert I've been observing regularly on a repro instance
using TLS:

repro: Connection.cxx:275: void resip::Connection::ensureWritable():
Assertion `!mOutstandingSends.empty()' failed.

I'm just wondering if anybody else has seen this or can comment on the
ensureWritable() logic before I look at it more closely?