Permit RunnerStack.peek to return null in case it is not in use (for example,...

Permit RunnerStack.peek to return null in case it is not in use (for example, from a workflow), so that we do not get an NPE when trying to run a task that asks for checkpoints.
https://trello.com/c/DoCMV6gB/31-runnerstack

core/src/main/java/hudson/model/Run.java

@@ -1518,7 +1518,11 @@ public abstract class Run <JobT extends Job<JobT,RunT>,RunT extends Run<JobT,Run
      * @see CheckPoint#report()
      */
     /*package*/ static void reportCheckpoint(@Nonnull CheckPoint id) {
-        RunnerStack.INSTANCE.peek().checkpoints.report(id);
+        Run<?,?>.RunExecution exec = RunnerStack.INSTANCE.peek();
+        if (exec == null) {
+            return;
+        }
+        exec.checkpoints.report(id);
     }
 
     /**
@@ -1526,7 +1530,11 @@ public abstract class Run <JobT extends Job<JobT,RunT>,RunT extends Run<JobT,Run
      */
     /*package*/ static void waitForCheckpoint(@Nonnull CheckPoint id, @CheckForNull BuildListener listener, @CheckForNull String waiter) throws InterruptedException {
         while(true) {
-            Run b = RunnerStack.INSTANCE.peek().getBuild().getPreviousBuildInProgress();
+            Run<?,?>.RunExecution exec = RunnerStack.INSTANCE.peek();
+            if (exec == null) {
+                return;
+            }
+            Run b = exec.getBuild().getPreviousBuildInProgress();
             if(b==null)     return; // no pending earlier build
             Run.RunExecution runner = b.runner;
             if(runner==null) {

core/src/main/java/hudson/model/RunnerStack.java

@@ -28,6 +28,7 @@ import hudson.model.Run.RunExecution;
 import java.util.Stack;
 import java.util.Map;
 import java.util.WeakHashMap;
+import javax.annotation.CheckForNull;
 
 /**
  * Keeps track of {@link RunExecution}s that are currently executing on the given thread
@@ -53,8 +54,19 @@ final class RunnerStack {
         if(s.isEmpty()) stack.remove(e);
     }
 
-    synchronized RunExecution peek() {
-        return stack.get(Executor.currentExecutor()).peek();
+    /**
+     * Looks up the currently running build, if known.
+     * @return a running build, or null if one has not been recorded
+     */
+    synchronized @CheckForNull RunExecution peek() {
+        Executor e = Executor.currentExecutor();
+        if (e != null) {
+            Stack<RunExecution> s = stack.get(e);
+            if (s != null && !s.isEmpty()) {
+                return s.peek();
+            }
+        }
+        return null;
     }
 
     static final RunnerStack INSTANCE = new RunnerStack();