[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/Proc.java

@@ -399,7 +399,7 @@ public abstract class Proc {
                     try {
                         byte[] buf = new byte[8192];
                         int len;
-                        while ((len = in.read(buf)) > 0) {
+                        while ((len = in.read(buf)) >= 0) {
                             out.write(buf, 0, len);
                             out.flush();
                         }