cli: do not allow default value for an empty address

Right now a bizarre error message can occur if an address flag was not
set: `wallet contains no account for 'AFmseVrdL9f9oyCzZefL9tG6UbvhPbdYzM'`.
This bug is 10x worse if it occures in `transfer` --from flag.

cli/flags/address.go

@@ -11,7 +11,10 @@ import (
 )
 
 // Address is a wrapper for Uint160 with flag.Value methods.
-type Address util.Uint160
+type Address struct {
+	IsSet bool
+	Value util.Uint160
+}
 
 // AddressFlag is a flag with type string
 type AddressFlag struct {
@@ -27,7 +30,7 @@ var (
 
 // String implements fmt.Stringer interface.
 func (a Address) String() string {
-	return address.Uint160ToString(util.Uint160(a))
+	return address.Uint160ToString(a.Value)
 }
 
 // Set implements flag.Value interface.
@@ -36,14 +39,24 @@ func (a *Address) Set(s string) error {
 	if err != nil {
 		return cli.NewExitError(err, 1)
 	}
-	*a = Address(addr)
+	a.IsSet = true
+	a.Value = addr
 	return nil
 }
 
 // Uint160 casts address to Uint160.
 func (a *Address) Uint160() (u util.Uint160) {
-	copy(u[:], a[:])
-	return
+	if !a.IsSet {
+		// It is a programmer error to call this method without
+		// checking if the value was provided.
+		panic("address was not set")
+	}
+	return a.Value
+}
+
+// IsSet checks if flag was set to a non-default value.
+func (f AddressFlag) IsSet() bool {
+	return f.Value.IsSet
 }
 
 // String returns a readable representation of this value

cli/wallet/wallet.go

@@ -198,6 +198,9 @@ func claimGas(ctx *cli.Context) error {
 	defer wall.Close()
 
 	addrFlag := ctx.Generic("address").(*flags.Address)
+	if !addrFlag.IsSet {
+		return cli.NewExitError("address was not provided", 1)
+	}
 	scriptHash := addrFlag.Uint160()
 	acc := wall.GetAccount(scriptHash)
 	if acc == nil {
@@ -395,6 +398,9 @@ func transferAsset(ctx *cli.Context) error {
 	defer wall.Close()
 
 	fromFlag := ctx.Generic("from").(*flags.Address)
+	if !fromFlag.IsSet {
+		return cli.NewExitError("'from' address was not provided", 1)
+	}
 	from := fromFlag.Uint160()
 	acc := wall.GetAccount(from)
 	if acc == nil {
@@ -431,7 +437,11 @@ func transferAsset(ctx *cli.Context) error {
 		return cli.NewExitError(err, 1)
 	}
 
-	toAddr := ctx.Generic("to").(*flags.Address).Uint160()
+	toFlag := ctx.Generic("to").(*flags.Address)
+	if !toFlag.IsSet {
+		return cli.NewExitError("'to' address was not provided", 1)
+	}
+	toAddr := toFlag.Uint160()
 	tx.AddOutput(&transaction.Output{
 		AssetID:    asset,
 		Amount:     amount,