Add bucketName checks for azure and s3 gateway in GetBucketInfo. (#4992)

Gateway interface implementations of GetBucketInfo() under
azure and s3 gateway did not perform any bucketname input
validation resulting in incorrect responses when the tests
are expecting InvalidBucketName.

Fixes #4983

cmd/gateway-azure-anonymous.go

@@ -133,10 +133,12 @@ func (a *azureObjects) AnonGetBucketInfo(bucket string) (bucketInfo BucketInfo,
 	if err != nil {
 		return bucketInfo, traceError(err)
 	}
+
 	bucketInfo = BucketInfo{
 		Name:    bucket,
 		Created: t,
 	}
+
 	return bucketInfo, nil
 }