Crash on ogre1 camera sensor cleanup

**Original report ([archived issue](https://osrf-migration.github.io/ignition-gh-pages/#!/ignitionrobotics/ign-rendering/issues/61)) by Ian Chen (Bitbucket: [Ian Chen](https://bitbucket.org/%7Beaa6fca5-6deb-43f6-907f-971c144735dd%7D/), GitHub: [iche033](https://github.com/iche033)).**

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# Description

When exiting ign-gazebo, cleaning up camera based sensors \(e.g. camera, lidar, etc\) causes segfaults. Ogre2 works fine though.

# Steps to reproduce

1. change ign-gazebo’s `sensors_demo.sdf` world file to use ogre1.x, i.e. change `ogre2` to `ogre`
2. launch ign-gazebo with `sensors_demo.sdf` world
3. exit ign-gazebo by closing the gui
4. ign-gazebo segfaults

\*\* Expected Behavior \*\*

ign-gazebo exist cleanly

\*\* Actual Behavior \*\*

ign-gazebo server segfaults

\*\* Reproduces how often \*\*

all the time

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Start by reproducing the crash with ign-gazebo and the sensors_demo.sdf world after switching ogre2 to ogre. Trace cleanup for camera-based sensors in the Ogre1 rendering path and verify that exiting the GUI no longer causes the ign-gazebo server to segfault.

Written by the indexing model from the issue text.