[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 (closed).

mentioned in issue flyspray/FS#1724 (closed)

mentioned in commit f9aede6e

Status changed to merged