move overloadCount out of constructor only if

Author: curin

sources/ClangSharp.PInvokeGenerator/PInvokeGenerator.VisitDecl.cs

@@ -508,7 +508,10 @@ private void VisitFunctionDecl(FunctionDecl functionDecl)
             var parent = cxxMethodDecl.Parent;
             Debug.Assert(parent is not null);
             name = GetRemappedCursorName(parent);
+        }
 
+        if (cxxMethodDecl is not null)
+        {
             overloadCount = GetOverloadCount(cxxMethodDecl);
         }