Merge bitcoin-core/gui#806: refactor: Misc int sign change fixes

05416422d3 refactor: Avoid implicit-integer-sign-change in processNewTransaction (MarcoFalke)
321f105d08 refactor: Avoid implicit-signed-integer-truncation-or-sign-change in FreedesktopImage (MarcoFalke)
6d8eecd33a refactor: Avoid implicit-integer-sign-change in createTransaction (MarcoFalke)

Pull request description:

  This is allowed by the language. However, the `integer` sanitizer complains about it. Thus, fix it, so that the `integer` sanitizer can be used in the future to catch unintended sign changes.

  Fixes #805.

ACKs for top commit:
  pablomartin4btc:
    tACK 05416422d3
  hebasto:
    ACK 05416422d3, I have reviewed the code and it looks OK.

Tree-SHA512: eaa941479bd7bee196eb8b31d93b8e1db122410cf62e8ec4cbbec35cfd14cc766081c3df5dd14a228e21ad2678d8b8ba0d2649e5934c994a90ae96d8b264b4ce

src/wallet/interfaces.cpp

@@ -286,7 +286,7 @@ public:
         if (!res) return util::Error{util::ErrorString(res)};
         const auto& txr = *res;
         fee = txr.fee;
-        change_pos = txr.change_pos ? *txr.change_pos : -1;
+        change_pos = txr.change_pos ? int(*txr.change_pos) : -1;
 
         return txr.tx;
     }