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

Merge branch 'bugfix/fcg-initial-direction' into 'master'

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.

See merge request [!226]

  [!226]: gitlab.dune-project.org/core/dune-istl/merge_requests/226