refactor: assert newly-created parent cache entry has zero memory usage

During `BatchWrite`, the parent entry is created under a guard that guarantees insertion, so the new `Coin` is default-constructed and empty.
Assert this invariant to document why there is no `cachedCoinsUsage` decrement before the assignment at this site.

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