fix: resolve underflow issue in price validation logic

Author: rube-de

contracts/contracts/sapphire/CrossChainPaymaster.sol

@@ -390,7 +390,7 @@ contract CrossChainPaymaster is
         // Validate token price
         if (tPrice <= 0) revert InvalidPrice(tPrice);
         if (tAnsweredIn < tRound) revert StalePrice(0, 0);
-        if (tUpdated == 0 || block.timestamp - tUpdated > stalenessThreshold) revert StalePrice(tUpdated, stalenessThreshold);
+        if (tUpdated == 0 || (block.timestamp > tUpdated && block.timestamp - tUpdated > stalenessThreshold)) revert StalePrice(tUpdated, stalenessThreshold);
 
         // Get ROSE/USD price (single aggregated feed from ROFL oracle)
         AggregatorV3Interface roseFeed = AggregatorV3Interface(roseUsdFeed);
@@ -399,7 +399,7 @@ contract CrossChainPaymaster is
         // Validate ROSE price
         if (rPrice <= 0) revert InvalidPrice(rPrice);
         if (rAnsweredIn < rRound) revert StalePrice(0, 0);
-        if (rUpdated == 0 || block.timestamp - rUpdated > stalenessThreshold) revert StalePrice(rUpdated, stalenessThreshold);
+        if (rUpdated == 0 || (block.timestamp > rUpdated && block.timestamp - rUpdated > stalenessThreshold)) revert StalePrice(rUpdated, stalenessThreshold);
 
         uint8 tokenDec = tokenDecimals[token];
         if (tokenDec == 0) tokenDec = 18;