Don't write to NULL after calling C natives returning void.

src/boot/be/abi.ml

@@ -149,7 +149,7 @@ type abi =
                                       -> unit);
 
     abi_emit_native_call_in_thunk: (Il.emitter
-                                    -> Il.cell                (* ret    *)
+                                    -> Il.cell option         (* ret    *)
                                       -> Common.nabi
                                         -> Il.operand         (* callee *)
                                           -> Il.operand array (* args   *)

src/boot/be/x86.ml

@@ -700,7 +700,7 @@ let emit_native_void_call
 
 let emit_native_call_in_thunk
     (e:Il.emitter)
-    (ret:Il.cell)
+    (ret:Il.cell option)
     (nabi:nabi)
     (fn:Il.operand)
     (args:Il.operand array)
@@ -734,9 +734,12 @@ let emit_native_call_in_thunk
     end;
 
     match ret with
-        Il.Reg (r, _) -> mov (word_at r) (ro eax)
-      | _ -> mov (rc edx) (c ret);
+        Some (Il.Reg (r, _)) ->
+          mov (word_at r) (ro eax)
+      | Some ret ->
+          mov (rc edx) (c ret);
           mov (word_at (h edx)) (ro eax)
+      | _ -> ()
 ;;
 
 

src/boot/me/trans.ml

@@ -4998,8 +4998,12 @@ let trans_visitor
                        libstr;
                        symstr |];
 
-                  abi.Abi.abi_emit_native_call_in_thunk (emitter())
-                    out nabi (Il.Cell f) args;
+                  abi.Abi.abi_emit_native_call_in_thunk
+                    (emitter())
+                    (if pointee_type out = Il.NilTy then None else Some out)
+                    nabi
+                    (Il.Cell f)
+                    args;
               end
 
           | _ -> bug ()