[bugfix] [solvers.hh] Set initial search direction of the flexible cg solver to zero

This adds a missing reset of the vector for the initial direction in the first iteration of the apply method.

In the current state the vector for the search direction contains the initial solution in the first iteration. Most of the standard preconditioners will add their result to the vector instead of replacing it. If the initial solution is not zero, this results in an incorrect first search direction.