fixed NPE.

git-svn-id: https://hudson.dev.java.net/svn/hudson/trunk/hudson/main@4259 71c3de6d-444a-0410-be80-ed276b4c234a

core/src/main/java/hudson/maven/MavenBuild.java

@@ -301,7 +301,8 @@ public class MavenBuild extends AbstractBuild<MavenModule,MavenBuild> {
                 return launcher.launchChannel(buildMavenCmdLine(listener).toCommandArray(),
                     out, null, envVars);
             } catch (IOException e) {
-                if(e.getMessage().contains("java: not found")) {
+                String msg = e.getMessage();
+                if(msg!=null && msg.contains("java: not found")) {
                     // diagnose issue #659
                     JDK jdk = getParent().getParent().getJDK();
                     if(jdk==null)