Add error handling to basic auth injector

Fixes a problem where basic auth was disabled and a nill pointer
was hit, causing a panic.

Signed-off-by: Alex Ellis <alexellis2@gmail.com>

gateway/handlers/basic_auth_injector.go

@@ -0,0 +1,20 @@
+// Copyright (c) OpenFaaS Author(s). All rights reserved.
+// Licensed under the MIT license. See LICENSE file in the project root for full license information.
+
+package handlers
+
+import (
+	"net/http"
+
+	"github.com/openfaas/faas-provider/auth"
+)
+
+type BasicAuthInjector struct {
+	Credentials *auth.BasicAuthCredentials
+}
+
+func (b BasicAuthInjector) Inject(r *http.Request) {
+	if r != nil && b.Credentials != nil {
+		r.SetBasicAuth(b.Credentials.User, b.Credentials.Password)
+	}
+}