qmf: fix undefined behavior in Vector::data
qmf: fix undefined behavior in Vector::data

Taking an address of the 0th element -- as in `&data_[0]` -- is undefined behavior for an empty vector `data_`.

Conceptually, attempting to access non-existing element `data_[0]` can happen regardless of whether its address is being taken or not.

Since C++14 is among requirements, a simple fix is to just use `std::vector<Double>::data`, which always does the right thing:
http://en.cppreference.com/w/cpp/container/vector/data

More details:
http://www.gotw.ca/gotw/074.htm
https://stackoverflow.com/questions/2728255/c-getting-the-address-of-the-start-of-an-stdvector
https://stackoverflow.com/questions/681725/what-do-i-get-from-front-of-empty-std-container
https://stackoverflow.com/questions/25419851/what-should-stdvectordata-return-if-the-vector-is-empty
quora/qmf