[VIAME] Fix undefined or incorrect behavior in darknet_trainer

[predicative]

Per rule 20 of the "Sequenced-before rules" at https://en.cppreference.com/w/cpp/language/eval_order, starting in C++17 the right-hand side of an assignment is evaluated before the left-hand side, which is not the desired behavior.

Prior to C++17, the behavior is, as I understand it, undefined.