seg fault

I have working ubuntu 18.04 / gazebo9 / ardupilot_gazebo / ros-melodic, but based on Pierre's setup (not swiftgus fork as per the readme here).
the planner segfaults as detailed:
```james@auturgy:~/Development/OctomapPlanner$ ./build/main_node
/home/james/Development/OctomapPlanner/src/OctomapServer.cpp OctomapServer: Line 49: Octomap Initalized
Info: InformedRRTstar: Space information setup was not yet called. Calling now.
Debug: InformedRRTstar: Planner range detected to be 5.688585
/home/james/Development/OctomapPlanner/src/Planner.cpp Planner: Line 91: Planner Initialized
/home/james/Development/OctomapPlanner/src/mavlink_comm.cpp MavlinkComm: Line 56: Mavlink Initialized
/home/james/Development/OctomapPlanner/src/arduplanner.cpp executePlan: Line 220: No new plan available, waiting for 1 seconds before next callback
/home/james/Development/OctomapPlanner/src/arduplanner.cpp executePlan: Line 220: No new plan available, waiting for 1 seconds before next callback
/home/james/Development/OctomapPlanner/src/arduplanner.cpp executePlan: Line 220: No new plan available, waiting for 1 seconds before next callback
/home/james/Development/OctomapPlanner/src/arduplanner.cpp executePlan: Line 220: No new plan available, waiting for 1 seconds before next callback
/home/james/Development/OctomapPlanner/src/arduplanner.cpp executePlan: Line 220: No new plan available, waiting for 1 seconds before next callback
/home/james/Development/OctomapPlanner/src/arduplanner.cpp replanCb: Line 229: Replanner called
/home/james/Development/OctomapPlanner/src/Planner.cpp setGoal: Line 134: Goal point set to: 5 1 1
/home/james/Development/OctomapPlanner/src/Planner.cpp setStart: Line 112: Start point set to: -0.12129 -0.112817 5.00172
/home/james/Development/OctomapPlanner/src/Planner.cpp replan: Line 180: Replanning
Warning: InformedRRTstar: Skipping invalid start state (invalid bounds)
at line 248 in /tmp/binarydeb/ros-melodic-ompl-1.4.0/src/ompl/base/src/Planner.cpp
Debug: InformedRRTstar: Discarded start state RealVectorState [-0.12129 -0.112817 5.00172]

Error: InformedRRTstar: There are no valid initial states!
at line 193 in /tmp/binarydeb/ros-melodic-ompl-1.4.0/src/ompl/geometric/planners/rrt/src/RRTstar.cpp
/home/james/Development/OctomapPlanner/src/Planner.cpp plan: Line 216: No solution found
/home/james/Development/OctomapPlanner/src/arduplanner.cpp replanCb: Line 246: New plan generated
Segmentation fault (core dumped)
james@auturgy:~/Development/OctomapPlanner$
```

Reproduce the failure by running ./build/main_node with the reported Ubuntu 18.04, Gazebo 9, ArduPilot Gazebo, and ROS Melodic setup. Start with src/Planner.cpp around plan and src/arduplanner.cpp around replanCb, where the log shows no solution before the crash. Done means the invalid-start/no-solution case no longer ends in a segmentation fault.

Written by the indexing model from the issue text.