fix inputstreamat bug

Author: chzyer

src/com/qiniu/utils/InputStreamAt.java

@@ -144,7 +144,7 @@ protected byte[] fileStreamRead(long offset, int length) throws IOException {
 		synchronized (data) {
 			mFileStream.seek(offset);
 			do {
-				read = mFileStream.read(data, totalRead, length);
+				read = mFileStream.read(data, totalRead, length - totalRead);
 				if (read <= 0) break;
 				totalRead += read;
 			} while (length > totalRead);