Empty custom matrix

[jcoupey]

We have a check in place for (custom) matrix size consistency. For example using "matrices": {"car": {"durations": [[0]]}} result in an adequate error if some location_index values are not zero. But the problem is that we somehow assume that a custom matrix is not empty. If it is, then based on its size in this check we behave as if no matrix was provided in input and try grabbing a non-existing routing wrapper.

With "matrices": {"car": {"durations": []}}:

$ vroom [...]
vroom: structures/vroom/input/input.cpp:576: vroom::Input::set_matrices(unsigned int)::<lambda(const std::vector<std::__cxx11::basic_string<char> >&)>: Assertion `rw != _routing_wrappers.end()' failed.
Aborted (core dumped)

This is really an edge case as there is no point in defining an empty custom matrix in input (probably why no-one reported this yet), but still we should handle this properly.

[jcoupey]

I'll fix this in #555 since I'm working on the whole matrix stuff there.

[jcoupey]

Fixed in #555