[fix panic] s3: fix object nil point (#6970)

**What this PR does / why we need it**:
fix a s3 panic.
```go
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0x14fec7b]

goroutine 6271245 [running]:
github.com/grafana/loki/pkg/storage/chunk/client.(*client).getChunk(_, {_, _}, _, {{0xb130cf997b272f99, {0xc05077e6a0, 0x1b}, 0x182d1aefb2d, 0x182d1b47984, 0x9fab6388}, ...})
/loki@v1.6.2-0.20220816085209-65ed7b60fbd4/pkg/storage/chunk/client/object_client.go:171 +0x2fb
```

pkg/storage/chunk/client/aws/s3_storage_client.go

@@ -381,7 +381,7 @@ func (a *S3ObjectClient) GetObject(ctx context.Context, objectKey string) (io.Re
 		if resp.ContentLength != nil {
 			size = *resp.ContentLength
 		}
-		if err == nil {
+		if err == nil && resp.Body != nil {
 			return resp.Body, size, nil
 		}
 		retries.Wait()

pkg/storage/chunk/client/object_client.go

@@ -168,6 +168,9 @@ func (o *client) getChunk(ctx context.Context, decodeContext *chunk.DecodeContex
 		return chunk.Chunk{}, errors.WithStack(err)
 	}
 
+	if readCloser == nil {
+		return chunk.Chunk{}, errors.New("object client getChunk fail because object is nil")
+	}
 	defer readCloser.Close()
 
 	// adds bytes.MinRead to avoid allocations when the size is known.