[Refactor] Fix a few compile time warnings

[Warrows]

Fixes the three following warnings when compiling:

wallet/rpcdump.cpp:31:6: warning: redundant redeclaration of ‘void EnsureWalletIsUnlocked(bool)’ in same scope [-Wredundant-decls]
 void EnsureWalletIsUnlocked(bool fAllowAnonOnly);
      ^~~~~~~~~~~~~~~~~~~~~~
In file included from wallet/rpcdump.cpp:10:
./rpc/server.h:181:13: note: previous declaration of ‘void EnsureWalletIsUnlocked(bool)’
 extern void EnsureWalletIsUnlocked(bool fAllowAnonOnly = false);
             ^~~~~~~~~~~~~~~~~~~~~~

qt/privacydialog.cpp: In member function ‘void PrivacyDialog::setBalance(const CAmount&, const CAmount&, const CAmount&, const CAmount&, const CAmount&, const CAmount&, const CAmount&, const CAmount&, const CAmount&)’:
qt/privacydialog.cpp:709:13: warning: variable ‘nLockedBalance’ set but not used [-Wunused-but-set-variable]
     CAmount nLockedBalance = 0;
             ^~~~~~~~~~~~~~

warning: nonnull argument 'this' compared to NULL [-Wnonnull-compare]

[CaveSpectre11]

utACK f6b506c

[random-zebra]

The first warning is addressed already in 955 (bad34b9).

The second is irrelevant, as 954 makes that code obsolete.

[Warrows]

Right, well there is still the third one. I'll rebase on top of these PR when they are merged.