[bugfix] fix broken assertion.

This patch fixes the errornous commits
e8d16384
1a2f4d0f

The introduced range checks were plain wring, as they neglected the nested structure of the
BlockVector. We now use `b.dim()` to get the flat size.

This fixes flyspray/FS#1724.
(cherry picked from commit 3aed940e)

Signed-off-by: Steffen Müthing <muething@dune-project.org>