Fix nil pointer panic on PUT /users/:id for a non-existent user - #444
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`users` is `map[int]*user{}`, so looking up a missing id returns a nil `*user`. `updateUser` did `users[id].Name = u.Name` directly, which panics with a nil pointer dereference on any PUT to an id that doesn't exist — easy to hit just by typing a wrong id in the cookbook's example requests.
Changed it to check for the user first (`existingUser, ok := users[id]`) and return a 404 if it's missing, matching how `getUser` and `deleteUser` already handle the same case elsewhere in this file.
Added tests for both the 404 path and the normal update path (create a user, update it, confirm the response reflects the new name).