quic: fix stream tx_fifo race condition

Type: fix

There is a race condition in when receiving TX from
a client application :
As egress_emit writes as much data as possible to
the stream, if during egress_emit the app writes
to the fifo, the data will be directly passed to
quicly. Then TX callback happens and triggers
a scheduler update telling quilcy the stream has
data to send. When the next egress_emit is called
and no more data has come, we have nothing to write,
we return len = 0 to quicly which breaks an assert if
a loss happens later on.

Change-Id: I47e00a14dfc9068b5dac7b5c090a89124aea004f
Signed-off-by: Nathan Skrzypczak <nathan.skrzypczak@gmail.com>