Fix possible NRE when disposing LeechCore in Vmm

Use null-conditional operator to safely dispose LeechCore, preventing a potential NullReferenceException if it is null (e.g., when Dispose is called from the constructor). Added a clarifying comment.

Author: lone7254

src/VmmSharpEx/Vmm.cs

@@ -195,7 +195,7 @@ private void Dispose(bool disposing)
         {
             if (disposing)
             {
-                LeechCore.Dispose();
+                LeechCore?.Dispose(); // Since this can be called from the ctor, Leechcore may be null here.
                 RefreshManager.UnregisterAll(this);
             }
             Vmmi.VMMDLL_Close(_handle);