Fix NPE on missing git data

Fix NPE on missing git data

Fix the following NPE:
!ENTRY org.eclipse.core.resources 4 2 2009-04-30 22:52:47.117
!MESSAGE Problems occurred when invoking code from plug-in: "org.eclipse.core.resources".
!STACK 0
java.lang.NullPointerException
    at org.spearce.egit.core.project.RepositoryMapping.getMapping(RepositoryMapping.java:235)
    at org.spearce.egit.ui.internal.decorators.GitLightweightDecorator$2.visit(GitLightweightDecorator.java:492)
    at org.eclipse.core.internal.events.ResourceDelta.accept(ResourceDelta.java:68)
    at org.eclipse.core.internal.events.ResourceDelta.accept(ResourceDelta.java:79)
    at org.eclipse.core.internal.events.ResourceDelta.accept(ResourceDelta.java:79)
    at org.eclipse.core.internal.events.ResourceDelta.accept(ResourceDelta.java:79)
    at org.eclipse.core.internal.events.ResourceDelta.accept(ResourceDelta.java:79)
    at org.eclipse.core.internal.events.ResourceDelta.accept(ResourceDelta.java:79)
    at org.eclipse.core.internal.events.ResourceDelta.accept(ResourceDelta.java:55)
    at org.spearce.egit.ui.internal.decorators.GitLightweightDecorator.resourceChanged(GitLightweightDecorator.java:477)
    at org.eclipse.core.internal.events.NotificationManager$2.run(NotificationManager.java:288)
    at org.eclipse.core.runtime.SafeRunner.run(SafeRunner.java:37)
    at org.eclipse.core.internal.events.NotificationManager.notify(NotificationManager.java:282)
    at org.eclipse.core.internal.events.NotificationManager.broadcastChanges(NotificationManager.java:148)
    at org.eclipse.core.internal.resources.Workspace.broadcastPostChange(Workspace.java:313)
    at org.eclipse.core.internal.resources.Workspace.endOperation(Workspace.java:1022)
    at org.eclipse.core.internal.resources.InternalWorkspaceJob.run(InternalWorkspaceJob.java:45)
    at org.eclipse.core.internal.jobs.Worker.run(Worker.java:55)

Signed-off-by: Daniel Cheng (aka SDiZ) <git@sdiz.net>
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>