Crash when executing render.py

[martinwinter-huawei]

https://github.com/xiaobiaodu/Mobile-GS/blob/main/submodules/diff-gaussian-rasterization/cuda_rasterizer/rasterizer_impl.cu line 438 is missing a return statement and hence crashes due to UB.
Returning P fixes the issue 👍

[xiaobiaodu]

Thank you for your help!