Make sure the vectors x and b handed to the 'apply' method have the correct size

This avoids strange bugs when using the setSubMatrix method: that method
automatically removes rows and columns from the matrix, and expects _the user_
to do the same for the x and b vectors.

(cherry picked from commit e8d16384)
Signed-off-by: Steffen Müthing <muething@dune-project.org>