Clean up nested scope in GetReservedDestination
Clean up nested scope in GetReservedDestination

Suggested https://github.com/bitcoin/bitcoin/pull/17304#discussion_r341194391
by Gregory Sanders <gsanders87@gmail.com>

Reason for keeping the `return true` `return false` verbosity is that more code
will be added after the ReserveKeyFromKeyPool() call before returning.

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