Fix exception in MatrixBuild.java:371

Happens when build has no parent:

15:06:14 FATAL: null
15:06:14 java.lang.NullPointerException
15:06:14    at hudson.matrix.MatrixBuild$MatrixBuildExecution.doRun(MatrixBuild.java:371)
15:06:14    at hudson.model.AbstractBuild$AbstractBuildExecution.run(AbstractBuild.java:580)
15:06:14    at hudson.model.Run.execute(Run.java:1575)
15:06:14    at hudson.matrix.MatrixBuild.run(MatrixBuild.java:304)
15:06:14    at hudson.model.ResourceController.execute(ResourceController.java:88)
15:06:14    at hudson.model.Executor.run(Executor.java:237)
15:06:14    at hudson.model.OneOffExecutor.run(OneOffExecutor.java:66)

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

@@ -368,7 +368,8 @@ public class MatrixBuild extends AbstractBuild<MatrixProject,MatrixBuild> {
                     final int n = getNumber();
                     for (MatrixConfiguration c : activeConfigurations) {
                         for (Item i : q.getItems(c)) {
-                            if (i.getAction(ParentBuildAction.class).parent==getBuild()) {
+                            ParentBuildAction a = i.getAction(ParentBuildAction.class);
+                            if (a!=null && a.parent==getBuild()) {
                                 q.cancel(i);
                                 logger.println(Messages.MatrixBuild_Cancelled(ModelHyperlinkNote.encodeTo(c)));
                             }