Update DefaultIOSystem.cpp (#5697)

- closes https://github.com/assimp/assimp/issues/5678

code/Common/DefaultIOSystem.cpp

@@ -93,6 +93,10 @@ static std::string WideToUtf8(const wchar_t *in) {
 // ------------------------------------------------------------------------------------------------
 // Tests for the existence of a file at the given path.
 bool DefaultIOSystem::Exists(const char *pFile) const {
+    if (pFile == nullptr) {
+        return false;
+    }
+        
 #ifdef _WIN32
     struct __stat64 filestat;
     if (_wstat64(Utf8ToWide(pFile).c_str(), &filestat) != 0) {