handler: Fix panic on GOARCH=386

Fixes #1047

pkg/handler/body_reader.go

@@ -19,10 +19,16 @@ import (
 // the error but this can instead be done in the handler.
 // In addition, the bodyReader keeps track of how many bytes were read.
 type bodyReader struct {
+	// bytesCounter is the first field to ensure that its properly aligned,
+	// otherwise we run into alignment issues on some 32-bit builds.
+	// See https://github.com/tus/tusd/issues/1047
+	// See https://pkg.go.dev/sync/atomic#pkg-note-BUG
+	// TODO: In the future we should move all of these values to the safe
+	// atomic.Uint64 type, which takes care of alignment automatically.
+	bytesCounter int64
 	ctx          *httpContext
 	reader       io.ReadCloser
 	err          error
-	bytesCounter int64
 	onReadDone   func()
 }