RemoveRedundantMaterials: Fix crash bug when unreferenced materials were destroyed. The logic only rebuilt the material list if there were redundant materials being removed. This is a clear bug as it left freed aiMaterial ptrs into the list and did not fix the scene->numMaterials to be correct, even when deleting materials. This crashed later on in the ComputeUVMappingsProcess that accessed the freed ptr.
Merge pull request #270 from jonnenauha/redundant-material-fix

RemoveRedundantMaterials: Fix crash bug when unreferenced materials are removed.