Use tmp struct and node id when converting subject

Break out the steps so it's clearer what is happening.

Verified that writing group subjects 4444, 5555, and 6666
properly got converted to node IDs 0xFFFFFFFFFFFF115C,
0xFFFFFFFFFFFF15B3, and 0xFFFFFFFFFFFF1A0A, and reading
them properly converted them back.

src/app/clusters/access-control-server/access-control-server.cpp

@@ -250,10 +250,11 @@ struct AccessControlEntryCodec
         {
             for (size_t i = 0; i < subjectCount; ++i)
             {
-                Subject subject;
-                ReturnErrorOnFailure(entry.GetSubject(i, subject.nodeId));
-                ReturnErrorOnFailure(Convert(subject.nodeId, subject));
-                subjectBuffer[i] = subject.nodeId;
+                NodeId subject;
+                ReturnErrorOnFailure(entry.GetSubject(i, subject));
+                Subject tmp;
+                ReturnErrorOnFailure(AccessControlEntryCodec::Convert(subject, tmp));
+                subjectBuffer[i] = tmp.nodeId;
             }
             staging.subjects.SetNonNull(subjectBuffer, subjectCount);
         }
@@ -302,9 +303,10 @@ struct AccessControlEntryCodec
             auto iterator = staging.subjects.Value().begin();
             while (iterator.Next())
             {
-                Subject subject = { .nodeId = iterator.GetValue(), .authMode = staging.authMode };
-                ReturnErrorOnFailure(Convert(subject, subject.nodeId));
-                ReturnErrorOnFailure(entry.AddSubject(nullptr, subject.nodeId));
+                Subject tmp = { .nodeId = iterator.GetValue(), .authMode = staging.authMode };
+                NodeId subject;
+                ReturnErrorOnFailure(Convert(tmp, subject));
+                ReturnErrorOnFailure(entry.AddSubject(nullptr, subject));
             }
             ReturnErrorOnFailure(iterator.GetStatus());
         }
@@ -393,10 +395,11 @@ CHIP_ERROR LogEntryChangedEvent(const AccessControl::Entry & entry, const Access
     {
         for (size_t i = 0; i < subjectCount; ++i)
         {
-            Subject subject;
-            ReturnErrorOnFailure(entry.GetSubject(i, subject.nodeId));
-            ReturnErrorOnFailure(AccessControlEntryCodec::Convert(subject.nodeId, subject));
-            subjectBuffer[i] = subject.nodeId;
+            NodeId subject;
+            ReturnErrorOnFailure(entry.GetSubject(i, subject));
+            Subject tmp;
+            ReturnErrorOnFailure(AccessControlEntryCodec::Convert(subject, tmp));
+            subjectBuffer[i] = tmp.nodeId;
         }
         staging.subjects.SetNonNull(subjectBuffer, subjectCount);
     }