[JENKINS-17728] Fixing another possible cause of an NPE.

Unlike the bug I originally reproduced, in which the parent had some builds
but the context in which the new build was scheduled omits a parent,
this seems to be due to a case in which there is no build record at all for the parent.
No idea how that could happen (getLastBuild should return even a running, failed, or aborted build),
but @treydock reports a stack trace in 1.509.2 which implies this.
So fixing null safety; will not prevent an exception but will report it more gracefully.

changelog.html

@@ -55,7 +55,9 @@ Upcoming changes</a>
 <!-- Record your changes in the trunk here. -->
 <div id="trunk" style="display:none"><!--=TRUNK-BEGIN=-->
 <ul class=image>
-  <li class=>
+  <li class=bug>
+    Fixed another possible cause of an NPE from MatrixConfiguration.newBuild.
+    (<a href="https://issues.jenkins-ci.org/browse/JENKINS-17728">issue 17728</a>)
 </ul>
 </div><!--=TRUNK-END=-->
 

core/src/main/java/hudson/matrix/MatrixConfiguration.java

@@ -239,6 +239,10 @@ public class MatrixConfiguration extends Project<MatrixConfiguration,MatrixRun>
             }
         }
 
+        if (lb == null) {
+            throw new IOException("cannot start a build of " + getFullName() + " since its parent has no builds at all");
+        }
+
         // for every MatrixRun there should be a parent MatrixBuild
         MatrixRun lastBuild = new MatrixRun(this, lb.getTimestamp());
 

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

@@ -135,6 +135,7 @@ import java.util.logging.Level;
 import java.util.logging.Logger;
 
 import static hudson.scm.PollingResult.*;
+import javax.annotation.CheckForNull;
 import static javax.servlet.http.HttpServletResponse.*;
 
 /**
@@ -1061,7 +1062,7 @@ public abstract class AbstractProject<P extends AbstractProject<P,R>,R extends A
     }
 
     @Override
-    public R getLastBuild() {
+    public @CheckForNull R getLastBuild() {
         return builds.newestBuild();
     }
 

core/src/main/java/jenkins/model/lazy/AbstractLazyLoadRunMap.java

@@ -46,6 +46,7 @@ import java.util.SortedMap;
 import java.util.TreeMap;
 import java.util.logging.Level;
 import java.util.logging.Logger;
+import javax.annotation.CheckForNull;
 
 import static jenkins.model.lazy.AbstractLazyLoadRunMap.Direction.*;
 import static jenkins.model.lazy.Boundary.*;
@@ -353,7 +354,7 @@ public abstract class AbstractLazyLoadRunMap<R> extends AbstractMap<Integer,R> i
      *      If ASC, finds the closest #M that satisfies M>=N.
      *      If DESC, finds the closest #M that satisfies M&lt;=N.
      */
-    public R search(final int n, final Direction d) {
+    public @CheckForNull R search(final int n, final Direction d) {
         Entry<Integer, BuildReference<R>> c = index.ceilingEntry(n);
         if (c!=null && c.getKey()== n) {
             R r = c.getValue().get();