Fix crash when returning void in an upcall

CHANGELOG.md

@@ -2,6 +2,8 @@
 All notable changes to this project will be documented in this file. This change log follows the conventions of [keepachangelog.com](http://keepachangelog.com/).
 
 ## [Unreleased]
+### Fixed
+- Void return types on upcalls would crash on serialization
 
 ## [0.1.251] - 2021-10-14
 ### Fixed

src/clj/coffi/mem.clj

@@ -417,6 +417,10 @@
       obj)
     (MemoryAddress/NULL)))
 
+(defmethod serialize* ::void
+  [_obj _type _scope]
+  nil)
+
 (defmulti serialize-into
   "Writes a serialized version of the `obj` to the given `segment`.