[FIXED JENKINS-8686]

As I mentioned in the comment, I doubt if this makes any behaviour change, but this seems like a sensible defensive measure that wouldn't hurt.

core/src/main/java/hudson/Util.java

@@ -451,7 +451,7 @@ public class Util {
     public static void copyStream(InputStream in,OutputStream out) throws IOException {
         byte[] buf = new byte[8192];
         int len;
-        while((len=in.read(buf))>0)
+        while((len=in.read(buf))>=0)
             out.write(buf,0,len);
     }
 
@@ -562,7 +562,7 @@ public class Util {
             byte[] buffer = new byte[1024];
             DigestInputStream in =new DigestInputStream(source,md5);
             try {
-                while(in.read(buffer)>0)
+                while(in.read(buffer)>=0)
                     ; // simply discard the input
             } finally {
                 in.close();