MFC r284941: Avoid a situation where we do not set persist timer after a zero window condition. If you send a 0-length packet, but there is data is the socket buffer, and neither the rexmt or persist timer is already set, then activate the persist timer.
MFC r284941:
Avoid a situation where we do not set persist timer after a zero window
condition.
If you send a 0-length packet, but there is data is the socket buffer, and
neither the rexmt or persist timer is already set, then activate the persist
timer.

PR:		192599
Approved by:	re (delphij)

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