GMRES: Improved robustness of pipelined implementation.

Changes:
 - correctly handle an all-zero right hand side
 - iteration count only reflects the true Krylov dimension used
   If orthogonality is lost, the extra basis vectors are not counted.
 - don't run into NaN on residual norm estimator if orthogonality is lost.

Reported-by: Andreas Rost (IHU GmbH)