subscription: fix setCompletionCallback

This was not being tested, and it was really broken. Now it is being
tested and it is no longer broken.

src/subscription.zig

@@ -174,12 +174,12 @@ pub const Subscription = opaque {
         self: *Subscription,
         comptime T: type,
         comptime callback: *const thunk.SimpleCallbackThunkSignature(T),
-        userdata: *T,
+        userdata: T,
     ) Error!void {
         return Status.fromInt(nats_c.natsSubscription_SetOnCompleteCB(
             @ptrCast(self),
-            thunk.makeSimpleCallbackThunk(callback),
-            @constCast(userdata),
+            thunk.makeSimpleCallbackThunk(T, callback),
+            @constCast(@ptrCast(userdata)),
         )).raise();
     }
 };

src/thunk.zig

@@ -57,7 +57,7 @@ pub fn makeSimpleCallbackThunk(
     comptime checkUserDataType(T);
     return struct {
         fn thunk(userdata: ?*anyopaque) callconv(.C) void {
-            const data: *T = if (userdata) |u| @alignCast(@ptrCast(u)) else unreachable;
+            const data: T = if (userdata) |u| @alignCast(@ptrCast(u)) else unreachable;
             callback(data);
         }
     }.thunk;