Thread.sleep(0) should not sleep indefinitely

Our implementation uses Object.wait(long) to implement Thread.sleep,
which had the side effect of interpreting zero as infinity.  However,
for Thread.sleep, zero just means zero.  I assume that doesn't mean
"don't sleep at all", though, or else the app wouldn't have called
Thread.sleep in the first place, so this patch sleeps for one
millisecond when zero is passed -- just enough to yield the processor
for a bit.  Thread.yield might be a better choice in this case, but I
assume the app would have called that directly if that's what it
wanted.

classpath/java/lang/Thread.java

@@ -156,6 +156,10 @@ public class Thread implements Runnable {
   }
 
   public static void sleep(long milliseconds) throws InterruptedException {
+    if (milliseconds <= 0) {
+      milliseconds = 1;
+    }
+
     Thread t = currentThread();
     if (t.sleepLock == null) {
       t.sleepLock = new Object();

src/classpath-openjdk.cpp

@@ -3309,6 +3309,10 @@ jvmSleep(Thread* t, uintptr_t* arguments)
 {
   jlong milliseconds; memcpy(&milliseconds, arguments, sizeof(jlong));
 
+  if (milliseconds <= 0) {
+    milliseconds = 1;
+  }
+
   if (threadSleepLock(t, t->javaThread) == 0) {
     object lock = makeJobject(t);
     set(t, t->javaThread, ThreadSleepLock, lock);