Fix NaN handling in currency conversion input validation

The functions didn't validate that credits and amountInCents are finite numbers.
If they were NaN or Infinity, Math.ceil(NaN * centsPerCredit) would return NaN.

Added Number.isFinite() checks to return 0 for invalid inputs.

Author: pavankumar-vh

common/src/util/currency.ts

@@ -8,6 +8,8 @@ export function convertCreditsToUsdCents(
   credits: number,
   centsPerCredit: number,
 ): number {
+  if (centsPerCredit <= 0) return 0
+  if (!Number.isFinite(credits)) return 0
   return Math.ceil(credits * centsPerCredit)
 }
 
@@ -21,5 +23,7 @@ export function convertStripeGrantAmountToCredits(
   amountInCents: number,
   centsPerCredit: number,
 ): number {
+  if (centsPerCredit <= 0) return 0
+  if (!Number.isFinite(amountInCents)) return 0
   return Math.floor(amountInCents / centsPerCredit)
 }