Document assumptions that are being made to avoid division by zero

src/policy/fees.cpp

@@ -503,6 +503,7 @@ void TxConfirmStats::removeTx(unsigned int entryHeight, unsigned int nBestSeenHe
         }
     }
     if (!inBlock && (unsigned int)blocksAgo >= scale) { // Only counts as a failure if not confirmed for entire period
+        assert(scale != 0);
         unsigned int periodsAgo = blocksAgo / scale;
         for (size_t i = 0; i < periodsAgo && i < failAvg.size(); i++) {
             failAvg[i][bucketindex]++;