NPE if change log cannot be fetched

Unfortunately we get an undocumented null-Reference from anyone of the elements. We should check every one!

[INFO] [INFO] Trace
[INFO] java.lang.NullPointerException
[INFO] at de.smartics.maven.plugin.buildmetadata.scm.maven.ScmAccessInfo.fetchChangeLog(ScmAccessInfo.java:187)
[INFO] at de.smartics.maven.plugin.buildmetadata.scm.maven.MavenScmRevisionNumberFetcher.fetchLatestRevisionNumber(MavenScmRevisionNumberFetcher.java:105)
[INFO] at de.smartics.maven.plugin.buildmetadata.RevisionHelper.provideScmBuildInfo(RevisionHelper.java:112)
[INFO] at de.smartics.maven.plugin.buildmetadata.BuildMetaDataMojo.provideScmBuildInfo(BuildMetaDataMojo.java:398)
[INFO] at de.smartics.maven.plugin.buildmetadata.BuildMetaDataMojo.execute(BuildMetaDataMojo.java:341)
[INFO] at org.apache.maven.plugin.DefaultPluginManager.executeMojo(DefaultPluginManager.java:483)
[INFO] at org.apache.maven.lifecycle.DefaultLifecycleExecutor.executeGoals(DefaultLifecycleExecutor.java:678)
[INFO] at org.apache.maven.lifecycle.DefaultLifecycleExecutor.executeGoalWithLifecycle(DefaultLifecycleExecutor.java:540)
[INFO] at org.apache.maven.lifecycle.DefaultLifecycleExecutor.executeGoal(DefaultLifecycleExecutor.java:519)
[INFO] at org.apache.maven.lifecycle.DefaultLifecycleExecutor.executeGoalAndHandleFailures(DefaultLifecycleExecutor.java:371)
[INFO] at org.apache.maven.lifecycle.DefaultLifecycleExecutor.executeTaskSegments(DefaultLifecycleExecutor.java:332)
[INFO] at org.apache.maven.lifecycle.DefaultLifecycleExecutor.execute(DefaultLifecycleExecutor.java:181)
[INFO] at org.apache.maven.DefaultMaven.doExecute(DefaultMaven.java:356)
[INFO] at org.apache.maven.DefaultMaven.execute(DefaultMaven.java:137)
[INFO] at org.apache.maven.cli.MavenCli.main(MavenCli.java:356)
[INFO] at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
[INFO] at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
[INFO] at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
[INFO] at java.lang.reflect.Method.invoke(Method.java:597)
[INFO] at org.codehaus.classworlds.Launcher.launchEnhanced(Launcher.java:315)
[INFO] at org.codehaus.classworlds.Launcher.launch(Launcher.java:255)
[INFO] at org.codehaus.classworlds.Launcher.mainWithExitCode(Launcher.java:430)
[INFO] at org.codehaus.classworlds.Launcher.main(Launcher.java:375)