Bugfix/import crashes

[umlaeute]

this fixes a few crashes when running assimp info on the test-models.

notably:

• LWOImporter: it seems there was a typo, swapping j for i (loop-vars rock!), which would create an out-of-bounds access on a vector
  • Closes: Bug: LWOImporter crashes on one of the test-files #4209
• RemoveRedundantMatsProcess: this crashed on an assertion (that there are actually materials left after removing all the redundant ones). i replaced the assertion by a throwing a DeadlyImportError
  • the documentation of BaseProcess says that the process() method should raise an ImportErrorException:

    assimp/code/Common/BaseProcess.h

    Lines 217 to 224 in 70f5cca

    	// -------------------------------------------------------------------
    	/** Executes the post processing step on the given imported data.
    	* A process should throw an ImportErrorException* if it fails.
    	* This method must be implemented by deriving classes.
    	* @param pScene The imported data to work at.
    	*/
    	virtual void Execute(aiScene *pScene) = 0;

    
    however, there is no such exception and other postprocessors used the DeadlyImportError, so i used that one as well (probably the docs should be updated)
    • Closes: Bug: Importer crashes on one of the MDL models #4224
    • Closes: Bug: Importer crashes on one of the glTF2 models #4225

[kimkulling]

Great work, thanks a lot for your contribution.