Don't use uninitialised stack memory if the sysctl variable
net.inet.tcp.hostcache.enable is set to 0.
The bug resulted in using possibly a too small MSS value or wrong
initial retransmission timer settings. Possibly the value used
for ssthresh was also wrong.

Submitted by: Richard Scheffenegger
Reviewed by: Cheng Cui, rgrimes@, tuexen@
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D23687