coins: assert just-created-coin has no dynamic memory usage

This was added for the two branches to be symmetric and to explain why this branch is missing a subtraction.

Co-authored-by: Andrew Toth <andrewstoth@gmail.com>

src/coins.cpp

@@ -195,6 +195,7 @@ bool CCoinsViewCache::BatchWrite(CoinsViewCacheCursor& cursor, const uint256 &ha
                 // and mark it as dirty.
                 itUs = cacheCoins.try_emplace(it->first).first;
                 CCoinsCacheEntry& entry{itUs->second};
+                assert(entry.coin.DynamicMemoryUsage() == 0);
                 if (cursor.WillErase(*it)) {
                     // Since this entry will be erased,
                     // we can move the coin into us instead of copying it