lnrpc: ensure parsing of the Amp flag for payments is consistent

In this commit, we fix an inconsistent in the API related to AMP
payments. When a payment request isn't specified, we require the `--amp`
flag on the CLI to make an AMP payment. However, for payment requests,
we don't require this flag. To fix this inconsistency, we now require
the `--amp` flag to _also_ be set for payment requests.

lnrpc/routerrpc/router_backend.go

@@ -940,6 +940,14 @@ func (r *RouterBackend) extractIntentFromSendRequest(
 
 		payAddr := payReq.PaymentAddr
 		if payReq.Features.HasFeature(lnwire.AMPOptional) {
+			// The opt-in AMP flag is required to pay an AMP
+			// invoice.
+			if !rpcPayReq.Amp {
+				return nil, fmt.Errorf("the AMP flag (--amp " +
+					"or SendPaymentRequest.Amp) must be " +
+					"set to pay an AMP invoice")
+			}
+
 			// Generate random SetID and root share.
 			var setID [32]byte
 			_, err = rand.Read(setID[:])