Fix LuaTable.getFreePos failing on GCed keys

We incorrectly strengthened the table key, which meant we didn't
distinguish between empty keys and GCed weak keys.

Fixes cc-tweaked/CC-Tweaked#1721

src/main/java/org/squiddev/cobalt/LuaTable.java

@@ -576,8 +576,7 @@ private void rehash(LuaValue extraKey, boolean mode) {
 	private int getFreePos() {
 		if (keys.length == 0) return -1;
 		while (lastFree >= 0) {
-			LuaValue last = key(lastFree--);
-			if (last == NIL) {
+			if (keys[lastFree--] == NIL) {
 				return lastFree + 1;
 			}
 		}
@@ -865,8 +864,6 @@ public void rawsetImpl(LuaValue key, LuaValue value) {
 			return;
 		}
 
-		// TODO: Check valid key here instead of at the call site!
-
 		do {
 			int node = getNode(key);
 			if (node == -1) node = newKey(key);

src/test/java/org/squiddev/cobalt/table/TableHashTest.java

@@ -266,4 +266,21 @@ public void testShrink() {
 		for (int i = 0; i < 8; i++) t.rawset("new_key_" + i, Constants.TRUE);
 		assertEquals(8, TableOperations.getHashLength(t));
 	}
+
+	@Test
+	public void testWeakCanBeReplaced() throws LuaError {
+		var t = new LuaTable();
+
+		var mt = new LuaTable();
+		mt.rawset("__mode", ValueFactory.valueOf("k"));
+		t.setMetatable(null, mt);
+
+		for (int j = 0; j < 16; j++) {
+			for (int i = 0; i < 256; i++) t.rawset(new LuaTable(), Constants.TRUE);
+			System.gc();
+
+			int hash = TableOperations.getHashLength(t);
+			if (hash > 512) throw new AssertionError("Expected hash to be <=512, but is " + hash);
+		}
+	}
 }