BUG/MAJOR: server: the "sni" directive could randomly cause trouble

BUG/MAJOR: server: the "sni" directive could randomly cause trouble

The "sni" server directive does some bad stuff on many occasions because
it works on a sample of type string and limits len to size-1 by hand. The
problem is that size used to be zero on many occasions before the recent
changes to smp_dup() and that it effectively results in setting len to -1
and writing the zero byte *before* the string (and not terminating the
string).

This patch makes use of the recently introduced smp_make_safe() to address
this issue.

This fix must be backported to 1.6.