MINOR: vars: make get_vars() allow the session to be null

In order to support manipulating variables from outside a session,
let's make get_vars() not assume that the session is always set.
diff --git a/src/vars.c b/src/vars.c
index 3390d24..3c19ec8 100644
--- a/src/vars.c
+++ b/src/vars.c
@@ -47,7 +47,7 @@
 	case SCOPE_PROC:
 		return &global.vars;
 	case SCOPE_SESS:
-		return &sess->vars;
+		return sess ? &sess->vars : NULL;
 	case SCOPE_CHECK: {
 			struct check *check = objt_check(sess->origin);