Add a LoanBroker invariant to compare CoverAvailable to balance

- Ensures that LoanBroker.CoverAvailable is never less than the
  pseudo-account balance for the Vault asset.

include/xrpl/protocol/STAmount.h

@@ -564,7 +564,7 @@ STAmount::clear()
 {
     // The -100 is used to allow 0 to sort less than a small positive values
     // which have a negative exponent.
-    mOffset = native() ? 0 : -100;
+    mOffset = integral() ? 0 : -100;
     mValue = 0;
     mIsNegative = false;
 }