Make sure void return types work as expected.

ffitest/so.c

@@ -77,3 +77,11 @@ big struct_big(int i, double d) {
     ret.c = ret.a + ret.b + 1000;
     return ret;
 }
+
+void void_fn(void) {
+    printf("void fn ran\n");
+}
+
+void void_ret_fn(int x) {
+    printf("void fn ran: %d\n", x);
+}

ffitest/test.janet

@@ -62,10 +62,17 @@
   [i d]
   (native-call struct-big-fn-pointer struct-big-fn-sig i d))
 
+(def void-fn-pointer (native-lookup module "void_fn"))
+(def void-fn-sig (native-signature :default :void))
+(defn void-fn
+  []
+  (native-call void-fn-pointer void-fn-sig))
+
 #
 # Call functions
 #
 
+(pp (void-fn))
 (pp (int-fn 10 20))
 (pp (double-fn 1.5 2.5 3.5))
 (pp (double-many 1 2 3 4 5 6))