Fixed a SEGFAULT in setPose reported in #18

Previously, calling setPose() on an interactive marker causes a SEGFAULT if applyChanges() was not called on the server at least once since the marker was created. I traced the actual SEGFAULT to the doSetPose function. The value of header passed from setPose() is invalid because, in this case, marker_context_it = marker_contexts_.end().

I added a check for this case and, if there is no marker is present, instead use the header from the pending update. I'm not 100% sure that this is the correct solution, so it would be good to get input from someone who is more familiar with the code base.