mutable-tree: Quiet clang-analyzer warning

mutable-tree: Quiet clang-analyzer warning

The warning here was about a use-after-free which is serious, but
I don't think it was reachable in practice, because we'd only
enter the second conditional if `contents_checksum != self->contents_checksum`,
yet we made them the same thing here.

Nevertheless, refactor the code to do a direct return in this
case because that's basically what happened before, and doing
so is a lot clearer.