* Adds nullptr checks and asserts to protect certain code paths
* Fixes wrong integer type in a printf call
* Adds const to const values
* Prevents integer overflow with explicit casts
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.