Merge bitcoin#11074: Assert that CWallet::SyncMetaData finds oldest transaction.

6c4042a Assert that CWallet::SyncMetaData finds oldest transaction. (Eelis)

Pull request description:

  Without this assert, the Clang static analyzer warns about subsequent dereferencing of copyFrom, because it can't be sure that it's not nullptr. See bitcoin#9573.

Tree-SHA512: 83cbcb32c52c94fcfefbc90ec7de2011dacd6bdb0da35adc401b8d8dda6a86de2fa0403e2158592268c2cf15eef4f3d887d98c90f1031d4735d5f4bf9dbc1d23

Author: laanwj

src/wallet/wallet.cpp

@@ -679,6 +679,9 @@ void CWallet::SyncMetaData(std::pair<TxSpends::iterator, TxSpends::iterator> ran
             copyFrom = &mapWallet[hash];
         }
     }
+
+    assert(copyFrom);
+
     // Now copy data from copyFrom to rest:
     for (TxSpends::iterator it = range.first; it != range.second; ++it)
     {