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.
diff --git a/src/ssl_sock.c b/src/ssl_sock.c
index 06d7cab..b681d63 100644
--- a/src/ssl_sock.c
+++ b/src/ssl_sock.c
@@ -3654,8 +3654,10 @@
 				global.ssl_be_keys_max = global.ssl_be_keys_per_sec.curr_ctr;
 
 			/* check if session was reused, if not store current session on server for reuse */
-			if (objt_server(conn->target)->ssl_ctx.reused_sess)
+			if (objt_server(conn->target)->ssl_ctx.reused_sess) {
 				SSL_SESSION_free(objt_server(conn->target)->ssl_ctx.reused_sess);
+				objt_server(conn->target)->ssl_ctx.reused_sess = NULL;
+			}
 
 			if (!(objt_server(conn->target)->ssl_ctx.options & SRV_SSL_O_NO_REUSE))
 				objt_server(conn->target)->ssl_ctx.reused_sess = SSL_get1_session(conn->xprt_ctx);