MFC r358023: Don't use uninitialized memory

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: rscheff
Reviewed by: Cheng Cui, rgrimes@, tuexen@
Differential Revision: https://reviews.freebsd.org/D23687