[BugFix] BlockVector: deallocate memory even if no copy of data is requested.

Previously memory was only releases in the case that the data
was copied. This resulted in a memory leak if no copying was
requested. This patch fixes this by moving the deallocation to
the correct scope and consequently closes flyspary #1326, see
http://www.dune-project.org/flyspray/index.php?do=details&task_id=1326