There's a code path where an input scene only has a single mesh.
In this case, if ProcessMesh returns 2, we delete the mesh and move on
with the postprocessing. UpdateMeshReferences is not called and so the
deleted mesh is left dangling in nodes.
In a later step (SplitMesh in my testing), it then tries to deference a
null pointer and we explode.
With `out = true`, we can now hit the DeadlyImportError instead of a
segfault.
In debug builds, there's no issues, but in release, an invalid mesh that
is deleted in FindInvalidDataProcess will be double free'd when the
parent scene is destroyed.
Changed copyright end year to 2020 in every reference "Copyright (c) 2006-XXXX, assimp team".
Changed copyright end year to 2020 in every reference "Copyright (c) 2006-XXXX, ASSIMP Development Team".
Changed copyright end year to 2020 in LICENCE.rtf.
Changed copyright end year in CMakeFiles.txt files and any other places referencing Assimp with a copyright start and end year.
