Referencing address of first element in vector instead of using data()

[Pindrought]

https://github.com/akuukka/quickhull/blob/master/Structs/VertexDataSource.hpp#L18

Using the following is problematic here since in the case of an empty vector this will cause an exception to be hit in debug mode.

 m_ptr(&vec[0])

Suggested fix:

 m_ptr(vec.data())

[akuukka]

Thanks! Fixed.