Fixed NPE during reporting (#318)

* Fixed potential NullPointerException.

There is evidence that the key sets of the sourceProxies and reporters
maps get inconsistent. I observe randomly failing builds that report the
following error:

```
FATAL: Cannot invoke "java.util.List.iterator()" because the return
value of "java.util.Map.get(Object)" is null
java.lang.NullPointerException: Cannot invoke
"java.util.List.iterator()" because the return value of
"java.util.Map.get(Object)" is null
	at hudson.maven.AbstractMavenBuilder.end(AbstractMavenBuilder.java:101)
	at hudson.maven.MavenModuleSetBuild$MavenModuleSetBuildExecution.doRun(MavenModuleSetBuild.java:883)
	at hudson.model.AbstractBuild$AbstractBuildExecution.run(AbstractBuild.java:526)
	at hudson.model.Run.execute(Run.java:1895)
	at hudson.maven.MavenModuleSetBuild.run(MavenModuleSetBuild.java:543)
	at hudson.model.ResourceController.execute(ResourceController.java:101)
	at hudson.model.Executor.run(Executor.java:442)
```

* Fixed indentation.

src/main/java/hudson/maven/AbstractMavenBuilder.java

@@ -98,7 +98,13 @@ protected AbstractMavenBuilder(BuildListener listener, Collection<MavenModule> m
     void end(Launcher launcher) throws IOException, InterruptedException {
         for (Map.Entry<ModuleName,ProxyImpl2> e : sourceProxies.entrySet()) {
             ProxyImpl2 p = e.getValue();
-            for (MavenReporter r : reporters.get(e.getKey())) {
+            ModuleName module = e.getKey();
+            List<MavenReporter> moduleReporters = reporters.get(module);
+            if (moduleReporters == null) {
+                // Safety: Key set of reporter and source list has become inconsistent.
+                continue;
+            }
+            for (MavenReporter r : moduleReporters) {
                 // we'd love to do this when the module build ends, but doing so requires
                 // we know how many task segments are in the current build.
                 r.end(p.owner(),launcher,listener);