When calling ->prepare_srv() callback for SSL server which
depends on global "nbthread" value, this latter was not already parsed,
so equal to 1 default value. This lead to bad memory accesses.
Thank you to Pieter (PiBa-NL) for having reported this issue and
for having provided a very helpful reg testing file to reproduce
this issue (reg-test/lua/b00002.*).
Must be backported to 1.8.
