LWSLoader: Fix out of bounds iterator access

Fix https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=38947
Fix #4222

code/AssetLib/LWS/LWSLoader.cpp

@@ -537,8 +537,8 @@ void LWSImporter::InternReadFile(const std::string &pFile, aiScene *pScene, IOSy
 
     // get file format version and print to log
     ++it;
-    
-    if ((*it).tokens[0].empty()) {
+
+    if (it == root.children.end() || (*it).tokens[0].empty()) {
         ASSIMP_LOG_ERROR("Invalid LWS file detectedm abort import.");
         return;
     }