C#: Fix logic for flow into property writes

Author: hvitved

csharp/ql/lib/semmle/code/csharp/dataflow/internal/DataFlowPrivate.qll

@@ -1165,8 +1165,14 @@ private module ArgumentNodes {
     ) {
       e1.(Argument).isArgumentOf(e2, _) and
       exactScope = false and
-      scope = e2 and
-      isSuccessor = true
+      isSuccessor = true and
+      if e2 instanceof PropertyWrite
+      then
+        exists(AssignableDefinition def |
+          def.getTargetAccess() = e2 and
+          scope = def.getExpr()
+        )
+      else scope = e2
     }
   }