Merge #11783: Fix shutdown in case of errors during initialization

Merge #11783: Fix shutdown in case of errors during initialization

d31e5c1 Fix shutdown in case of errors during initialization (Wladimir J. van der Laan)

Pull request description:

  PR #10286 introduced a few steps which are not robust to early shutdown in initialization.

  Stumbled upon this with #11781, not sure if there are other scenarios that can trigger it, but it's good to harden against this in any case.

  E.g.
  ```
  $ src/bitcoind  -debuglogfile=/dfdf
  Error: Could not open debug log file /dfdf
  Program received signal SIGSEGV, Segmentation fault.
  UnregisterValidationInterface (pwalletIn=0x0) at /.../bitcoin/src/validationinterface.cpp:82
  82          g_signals.m_internals->BlockChecked.disconnect(boost::bind(&CValidationInterface::BlockChecked, pwalletIn, _1, _2));
  (gdb) bt
  #0  UnregisterValidationInterface (pwalletIn=0x0) at /.../bitcoin/src/validationinterface.cpp:82
  #1  0x00005555555a11fc in Shutdown () at /.../bitcoin/src/init.cpp:196
  #2  0x00005555555961cc in AppInit (argc=<optimized out>, argv=<optimized out>) at /.../bitcoin/src/bitcoind.cpp:183
  #3  0x0000555555596249 in main (argc=0, argv=0x555555ecf200) at /.../bitcoin/src/bitcoind.cpp:19
  ```

Tree-SHA512: 7dd9570a9803514a17781bfadf1edde47e96df4e852cce2f423cab422e005fb94d44e777af1a6ea5167b04a4d889e848ae7a61a7e0e94232247ddea32ee70fc8