Merge bitcoin/bitcoin#29510: wallet: `getrawchangeaddress` and `getnewaddress` failures should not affect keypools for descriptor wallets

e073f1dfda7a2a2cb2be9fe2a1d576f122596021 test: make sure keypool sizes do not change on `getrawchangeaddress`/`getnewaddress` failures (UdjinM6)
367bb7a80cc71130995672c853d4a6e0134721d6 wallet: Avoid updating `ReserveDestination::nIndex` when `GetReservedDestination` fails (UdjinM6)

Pull request description:

  I think the expected behaviour of `getrawchangeaddress` and `getnewaddress` RPCs is that their failure should not affect keypool in any way. At least that's how legacy wallets work, you can confirm this behaviour by running `wallet_keypool.py --legacy-wallet` on master with e073f1dfda7a2a2cb2be9fe2a1d576f122596021 applied on top. However running `wallet_keypool.py --descriptors` on the same commit results in the following failure:
  ```
    File "/path/to/bitcoin/test/functional/test_framework/test_framework.py", line 131, in main
      self.run_test()
    File "/path/to/bitcoin/test/functional/wallet_keypool.py", line 114, in run_test
      assert_equal(kp_size_before, kp_size_after)
    File "/path/to/bitcoin/test/functional/test_framework/util.py", line 57, in assert_equal
      raise AssertionError("not(%s)" % " == ".join(str(arg) for arg in (thing1, thing2) + args))
  AssertionError: not([18, 24] == [19, 24])
  ```

  This happens because we pass `nIndex` (which is a class member) into `GetReservedDestination` and since it's passed by reference we get an updated value back, so `nIndex` won't be equal `-1` anymore, no matter if the function failed or succeeded. This means that `ReturnDestination` (called by dtor of `ReserveDestination`) will try to return something we did not actually reserve.

  The fix is to simply use a temporary variable instead of a class member and only update `nIndex` when `op_address` actually has value, basically do it the same way we do for other class members (`address` and `fInternal`) already.

ACKs for top commit:
  achow101:
    ACK e073f1dfda7a2a2cb2be9fe2a1d576f122596021
  josibake:
    ACK https://github.com/bitcoin/bitcoin/pull/29510/commits/e073f1dfda7a2a2cb2be9fe2a1d576f122596021

Tree-SHA512: 1128288a60dd4d8f306ef6f7ac66cdfeae3c9cc35c66ecada2d78fa61ac759f2a757b70fc3976ba8b5081200942b58dfabc184c01ccf911af40ba8c145344651

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