Fix work with weak tables.

```
a = {}; setmetatable(a, {__mode = 'vk'});
a[1], a[2], a[3] = {}, {}, {};
for k, v in pairs(a) do
	print(k, v)
end
print('ok')
```

src/core/org/luaj/vm2/LuaTable.java

@@ -472,7 +472,8 @@ public class LuaTable extends LuaValue implements Metatable {
 				}
 			}
 			if ( checkLoadFactor() ) {
-				if ( key.isinttype() && key.toint() > 0 ) {
+				if ( (m_metatable == null || !(m_metatable.useWeakKeys() || m_metatable.useWeakValues()))
+					&& key.isinttype() && key.toint() > 0 ) {
 					// a rehash might make room in the array portion for this key.
 					rehash( key.toint() );
 					if ( arrayset(key.toint(), value) )