BUG/MEDIUM: ssl: properly reset the reused_sess during a forced handshake

BUG/MEDIUM: ssl: properly reset the reused_sess during a forced handshake

We have a bug when SSL reuse is disabled on the server side : we reset
the context but do not set it to NULL, causing a multiple free of the
same entry. It seems like this bug cannot appear as-is with the current
code (or the conditions to get it are not obvious) but it did definitely
strike when trying to fix another bug with the SNI which forced a new
handshake.

This fix should be backported to 1.7, 1.6 and 1.5.