Refactor ZCharArray - clear array on disposal

`ZCharArray` now ensures the buffer is always returned to the pool with `clearArray: true`.

Author: axunonb

src/SmartFormat/ZString/ZCharArray.cs

@@ -273,7 +273,7 @@ public void Dispose()
     {
         if (_isDisposed) return;
 
-        Pool.Return(_bufferArray, clearArray: true);
-        _isDisposed = true;
+        Pool.Return(_bufferArray!, clearArray: true);
+        _bufferArray = null;
     }
 }