BUG/MINOR: vars: use sess and not s->sess in action_store()

This causes the stream to be dereferenced when not needed. It will
cause trouble when variables are used outside of a stream.
diff --git a/src/vars.c b/src/vars.c
index 56fade5..b22c3bf 100644
--- a/src/vars.c
+++ b/src/vars.c
@@ -507,7 +507,7 @@
 
 	/* Process the expression. */
 	memset(&smp, 0, sizeof(smp));
-	if (!sample_process(px, s->sess, s, dir|SMP_OPT_FINAL,
+	if (!sample_process(px, sess, s, dir|SMP_OPT_FINAL,
 	                    rule->arg.vars.expr, &smp))
 		return ACT_RET_CONT;