b2: fix streaming chunked files an exact multiple of chunk size

Before this change, streaming files an exact multiple of the chunk
size would cause rclone to attempt to stream a 0 sized chunk which was
rejected by the b2 servers.

This bug was noticed by the new integration tests for chunked streaming.

backend/b2/upload.go

@@ -417,7 +417,13 @@ func (up *largeUpload) Stream(ctx context.Context, initialUploadBlock *pool.RW)
 		} else {
 			n, err = io.CopyN(rw, up.in, up.chunkSize)
 			if err == io.EOF {
-				fs.Debugf(up.o, "Read less than a full chunk, making this the last one.")
+				if n == 0 {
+					fs.Debugf(up.o, "Not sending empty chunk after EOF - ending.")
+					up.f.putRW(rw)
+					break
+				} else {
+					fs.Debugf(up.o, "Read less than a full chunk %d, making this the last one.", n)
+				}
 				hasMoreParts = false
 			} else if err != nil {
 				// other kinds of errors indicate failure