Make sure to only call memcpy when not nullptr
Make sure to only call memcpy when not nullptr

Fixes test JxlTest.RoundtripAnimationPatches (using ubsan)

  libjxl/lib/jxl/blending.cc:81:13: runtime error: null pointer passed as argument 1, which is declared to never be null

libjxl/libjxl,libjxl/libjxl,libjxl/libjxl,libjxl/libjxl,libjxl/libjxl,libjxl/libjxl,libjxl/libjxl,libjxl/libjxl