Fix a crash on splash screen when wallet fails to load

[UdjinM6]

Discovered by @xdustinface:

Happens because this unique_ptr https://github.com/dashpay/dash/blob/develop/src/wallet/wallet.cpp#L5102 gets added to the list of wallets in splashscreen inside https://github.com/dashpay/dash/blob/develop/src/wallet/wallet.cpp#L5104 which calls SplashScreen::ConnectWallet due to the uiInterface if general wallet loading fails. But if the further wallet validation in CreateWalletFromFile fails the unique ptr is deallocated but the splashscreen still hold a reference to its former address. Then it tries to disconnect the signal here https://github.com/dashpay/dash/blob/develop/src/qt/splashscreen.cpp#L182 and crashes.

This is fixed in bitcoin#13063 which depends on a HUGE gui/node/wallet separation PR bitcoin#10244 and there is no way we backport these to 0.16. However the only thing that uses ShowProgress on splash screen is ScanForWalletTransactions(), so we can just call uiInterface.LoadWallet() right next to it (instead of doing it in CWallet:LoadWallet()) as a workaround.

[xdustinface]

ACK

[PastaPastaPasta]

hmm, what if we only merge this to 0.16 and NOT to 17, since I'm going to be backporting 13063 soon:tm: anyway?

[UdjinM6]

13063 moved uiInterface.LoadWallet() out of CWallet:LoadWallet() too, so merge conflicts should be minimal/trivial IMO. (or you could simply revert this before applying the backported commit)

[PastaPastaPasta]

Okay, utACK