R1:13c8963dab27

Fix a Mercurial wire protocol parser issue when we receive a length frame before any data

Summary:
Depends on D18856. Ref T13036. See PHI275. When we receive a length frame but the buffer doesn't have any data yet, we currently emit a pointless 0-length data frame on the channel.

For normal chatter this is harmless/valid, but it causes problems when a channel has transitioned into bundle2 mode (probably it indicates "end of stream")?

In any case, it's never helpful, so if we're about to read a data…