Never directly dereference AWS SDK returned pointers

The SDK provides helper functions when you want an empty value if
the actual one is nil so we can just wrap all of them to avoid getting
bit.

Fixes benbjohnson#556
Replaces benbjohnson#555

Also fixes another panic when getting a delete error without key.

s3/replica_client.go

@@ -161,7 +161,7 @@ func (c *ReplicaClient) Generations(ctx context.Context) ([]string, error) {
 		internal.OperationTotalCounterVec.WithLabelValues(ReplicaClientType, "LIST").Inc()
 
 		for _, prefix := range page.CommonPrefixes {
-			name := path.Base(*prefix.Prefix)
+			name := path.Base(aws.StringValue(prefix.Prefix))
 			if !litestream.IsGenerationName(name) {
 				continue
 			}
@@ -292,7 +292,7 @@ func (c *ReplicaClient) SnapshotReader(ctx context.Context, generation string, i
 		return nil, err
 	}
 	internal.OperationTotalCounterVec.WithLabelValues(ReplicaClientType, "GET").Inc()
-	internal.OperationBytesCounterVec.WithLabelValues(ReplicaClientType, "GET").Add(float64(*out.ContentLength))
+	internal.OperationBytesCounterVec.WithLabelValues(ReplicaClientType, "GET").Add(float64(aws.Int64Value(out.ContentLength)))
 
 	return out.Body, nil
 }
@@ -386,7 +386,7 @@ func (c *ReplicaClient) WALSegmentReader(ctx context.Context, pos litestream.Pos
 		return nil, err
 	}
 	internal.OperationTotalCounterVec.WithLabelValues(ReplicaClientType, "GET").Inc()
-	internal.OperationBytesCounterVec.WithLabelValues(ReplicaClientType, "GET").Add(float64(*out.ContentLength))
+	internal.OperationBytesCounterVec.WithLabelValues(ReplicaClientType, "GET").Add(float64(aws.Int64Value(out.ContentLength)))
 
 	return out.Body, nil
 }
@@ -527,7 +527,7 @@ func (itr *snapshotIterator) fetch() error {
 		internal.OperationTotalCounterVec.WithLabelValues(ReplicaClientType, "LIST").Inc()
 
 		for _, obj := range page.Contents {
-			key := path.Base(*obj.Key)
+			key := path.Base(aws.StringValue(obj.Key))
 			index, err := litestream.ParseSnapshotPath(key)
 			if err != nil {
 				continue
@@ -536,7 +536,7 @@ func (itr *snapshotIterator) fetch() error {
 			info := litestream.SnapshotInfo{
 				Generation: itr.generation,
 				Index:      index,
-				Size:       *obj.Size,
+				Size:       aws.Int64Value(obj.Size),
 				CreatedAt:  obj.LastModified.UTC(),
 			}
 
@@ -630,7 +630,7 @@ func (itr *walSegmentIterator) fetch() error {
 		internal.OperationTotalCounterVec.WithLabelValues(ReplicaClientType, "LIST").Inc()
 
 		for _, obj := range page.Contents {
-			key := path.Base(*obj.Key)
+			key := path.Base(aws.StringValue(obj.Key))
 			index, offset, err := litestream.ParseWALSegmentPath(key)
 			if err != nil {
 				continue
@@ -640,7 +640,7 @@ func (itr *walSegmentIterator) fetch() error {
 				Generation: itr.generation,
 				Index:      index,
 				Offset:     offset,
-				Size:       *obj.Size,
+				Size:       aws.Int64Value(obj.Size),
 				CreatedAt:  obj.LastModified.UTC(),
 			}
 
@@ -763,9 +763,9 @@ func deleteOutputError(out *s3.DeleteObjectsOutput) error {
 	case 0:
 		return nil
 	case 1:
-		return fmt.Errorf("deleting object %s: %s - %s", *out.Errors[0].Key, *out.Errors[0].Code, *out.Errors[0].Message)
+		return fmt.Errorf("deleting object %s: %s - %s", aws.StringValue(out.Errors[0].Key), aws.StringValue(out.Errors[0].Code), aws.StringValue(out.Errors[0].Message))
 	default:
 		return fmt.Errorf("%d errors occurred deleting objects, %s: %s - (%s (and %d others)",
-			len(out.Errors), *out.Errors[0].Key, *out.Errors[0].Code, *out.Errors[0].Message, len(out.Errors)-1)
+			len(out.Errors), aws.StringValue(out.Errors[0].Key), aws.StringValue(out.Errors[0].Code), aws.StringValue(out.Errors[0].Message), len(out.Errors)-1)
 	}
 }