fix a problem w/ zero byte writes to the tunnel device.  It would bypass the loop and not set an error, so we would then try to access an invalid mbuf...
fix a problem w/ zero byte writes to the tunnel device.  It would bypass
the loop and not set an error, so we would then try to access an invalid
mbuf...

PR:		12780
Submitted by:	bright@rush.net aka zb^3

a new record in length a pr was open... only about a half hour...

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