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The new general convex path shadow code was checking for duplicate vertices without actually checking if the vectors contained any vertices. Thus, <vector>.back() was being called on empty vectors, which is bad. This led to a crash in G3 as their code is being run with a vector implementation that protects against this, but apparently we do not have such a vector implementation in our local building and testing. No tests because this covered by existing test cases and a new FML_DCHECK. <!-- Thanks for filing a pull request! Reviewers are typically assigned within a week of filing a request. To learn more about code review, see our documentation on Tree Hygiene: https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md --> Fixes https://github.com/flutter/flutter/issues/180827 Impacted users: Any user who uses shadows (pretty much every Flutter app) on a platform that has address sanitization built into its `std::vector` implementation, if they use Impeller (always used on iOS and the default on Android) Impact Description: Every shadow render operation in Impeller backend will access `back()` on an empty `std::vector` which will crash immediately if the vector is using sanitization on that platform. Workaround: Potentially look into disabling address sanitization? On platforms that still support Skia, it can be used as a workaround, but that is not an option on iOS Risk: The fix was obvious and non-controversial. Test Coverage: Existing tests cover this case really well and there is now an FML_DCHECK (engine assert) to check if we trigger the condition under testing. Validation: The original issue indicates an app that will trigger the sanitization crash, but only in G3. None of the platforms we support directly through flutter perform this sanitization. **Note**: The Flutter team is currently trialing the use of [Gemini Code Assist for GitHub](https://developers.google.com/gemini-code-assist/docs/review-github-code). Comments from the `gemini-code-assist` bot should not be taken as authoritative feedback from the Flutter team. If you find its comments useful you can update your code accordingly, but if you are unsure or disagree with the feedback, please feel free to wait for a Flutter team member's review for guidance on which automated comments should be addressed.