Fix offset rollback during serialization for derived types in AOT mode

I'm not positive if it was actually variable boxing, but I was getting an issue where
the value of offset would roll back to the value it was at the last time the specific
SerializeDelegate for that type was used, which would then blow up on deserialization.

Moving the args definition inside the delegate body fixes it

src/Ceras/Formatters/ReferenceFormatter.cs

@@ -536,9 +536,9 @@ static SerializeDelegate<T> CreateSpecificSerializerDispatcher_Aot(Type type, IF
  }
 
  // Can't call directly, need to invoke through reflection so T gets casted up/down correctly.
- var args = new object[3];
  return (ref byte[] buffer, ref int offset, T value) =>
  {
+ var args = new object[3];
  args[0] = buffer;
  args[1] = offset;
  args[2] = value;
@@ -559,10 +559,9 @@ static DeserializeDelegate<T> CreateSpecificDeserializerDispatcher_Aot(Type type
  // return (ref byte[] buffer, ref int offset, T value) => { f.Serialize(ref buffer, ref offset, value); };
  }
 
-
- var args = new object[3];
  return new DeserializeDelegate<T>((byte[] buffer, ref int offset, ref T value) =>
  {
+ var args = new object[3];
  args[0] = buffer;
  args[1] = offset;
  args[2] = value;