numpy::from_data calls vector::front and crashes (or has UB) when called with a scalar object #526

In ndarray.cpp, the function from_data_impl calls &shape.front() and &strides.front() for possibly empty vectors. This is, at best, undefined behavior. At worst, it triggers a crash (observed on Redhat/rocky 8&9, where I'm assuming boost::python packages were built with -D_GLIBCXX_DEBUG).

RECOMMEND: change &vec.front() to vec.data() to avoid UB and potential crashes.