disallow a computation allowance of 0 (#5541)

src/test/app/Escrow_test.cpp

@@ -1970,6 +1970,13 @@ struct Escrow_test : public beast::unit_test::suite
                 ter(tefWASM_FIELD_NOT_INCLUDED));
         }
 
+        {
+            // ComputationAllowance value of 0
+            env(escrow::finish(carol, alice, seq),
+                escrow::comp_allowance(0),
+                ter(temBAD_LIMIT));
+        }
+
         {
             // not enough fees
             // This function takes 4 gas

src/xrpld/app/tx/detail/Escrow.cpp

@@ -743,6 +743,10 @@ EscrowFinish::preflight(PreflightContext const& ctx)
 
     if (auto const allowance = ctx.tx[~sfComputationAllowance]; allowance)
     {
+        if (*allowance == 0)
+        {
+            return temBAD_LIMIT;
+        }
         if (*allowance > ctx.app.config().FEES.extension_compute_limit)
         {
             JLOG(ctx.j.debug())