Update scene.h

Add some more checks against nullptr dereferecnes.

include/assimp/scene.h

@@ -5,8 +5,6 @@ Open Asset Import Library (assimp)
 
 Copyright (c) 2006-2021, assimp team
 
-
-
 All rights reserved.
 
 Redistribution and use of this software in source and binary forms,
@@ -402,15 +400,23 @@ struct aiScene
 
     //! Returns an embedded texture and its index
     std::pair<const aiTexture*, int> GetEmbeddedTextureAndIndex(const char* filename) const {
+        if(nullptr==filename) {
+            return std::make_pair(nullptr, -1);
+        }
         // lookup using texture ID (if referenced like: "*1", "*2", etc.)
         if ('*' == *filename) {
             int index = std::atoi(filename + 1);
-            if (0 > index || mNumTextures <= static_cast<unsigned>(index))
+            if (0 > index || mNumTextures <= static_cast<unsigned>(index)) {
                 return std::make_pair(nullptr, -1);
+            }
             return std::make_pair(mTextures[index], index);
         }
         // lookup using filename
         const char* shortFilename = GetShortFilename(filename);
+        if (nullptr == shortFilename) {
+            return std::make_pair(nullptr, -1);
+        }
+
         for (unsigned int i = 0; i < mNumTextures; i++) {
             const char* shortTextureFilename = GetShortFilename(mTextures[i]->mFilename.C_Str());
             if (strcmp(shortTextureFilename, shortFilename) == 0) {