Allow non-varargs to get wrapped with sync

This early return skipped logic below to wrap the invoker with
synchronization.

src/main/java/jnr/ffi/provider/jffi/DefaultInvokerFactory.java

@@ -138,7 +138,7 @@ public Invoker createInvoker(Method method) {
                 marshallers[i] = getMarshaller(parameterTypes[i]);
             }
 
-            return new DefaultInvoker(runtime, library, function, functionInvoker, marshallers);
+            invoker = new DefaultInvoker(runtime, library, function, functionInvoker, marshallers);
         }
 
         //