Merge #13683: wallet: Introduce assertion to document the assumption that cache and cache_used are always set in tandem

d06330396f wallet: Avoid potential null pointer dereference in CWalletTx::GetAvailableCredit(...) (practicalswift)

Pull request description:

  Avoid potential null pointer dereference in `CWalletTx::GetAvailableCredit(...)`.

  Introduced in 4279da4785.

Tree-SHA512: 92d1da9682f0bab11f6f96395ca30549331b0a056cbceb7e1a7f98b3d17d10082aaeed0907cafd46f4164b0e0f4b77e01f78bfd7d24e0503a66c6942ae842aa5

src/wallet/wallet.cpp

@@ -1977,6 +1977,7 @@ CAmount CWalletTx::GetAvailableCredit(bool fUseCache, const isminefilter& filter
 
     if (cache) {
         *cache = nCredit;
+        assert(cache_used);
         *cache_used = true;
     }
     return nCredit;