Avoid unclear {it = ++it;}

Author: sipa

src/test/coins_tests.cpp

@@ -55,7 +55,7 @@ class CCoinsViewTest : public CCoinsView
 
     bool BatchWrite(CCoinsMap& mapCoins, const uint256& hashBlock, bool erase = true) override
     {
-        for (CCoinsMap::iterator it = mapCoins.begin(); it != mapCoins.end(); it = erase ? mapCoins.erase(it) : ++it) {
+        for (CCoinsMap::iterator it = mapCoins.begin(); it != mapCoins.end(); it = erase ? mapCoins.erase(it) : std::next(it)) {
             if (it->second.flags & CCoinsCacheEntry::DIRTY) {
                 // Same optimization used in CCoinsViewDB is to only write dirty entries.
                 map_[it->first] = it->second.coin;