BUG/MINOR: lua: in some case a sample may remain undefined

When we transform a top stack entry in sample, the empty stack case
is not handled. This patch fix this behavior.
diff --git a/src/hlua.c b/src/hlua.c
index 5357b32..f7ce3a5 100644
--- a/src/hlua.c
+++ b/src/hlua.c
@@ -508,6 +508,8 @@
 	case LUA_TFUNCTION:
 	case LUA_TTHREAD:
 	case LUA_TLIGHTUSERDATA:
+	case LUA_TNONE:
+	default:
 		smp->data.type = SMP_T_BOOL;
 		smp->data.u.sint = 0;
 		break;