Fix a stack corruption when xbt_dynar_remove_at was used with object larger than...

Fix a stack corruption when xbt_dynar_remove_at was used with object larger than 8bytes. It was the case in gras_procdata_exit: we had a by 4 overflow. This leaded to segfault when using optimization on gcc 4.x since %ebx is stored right after our data in that case. YEAH, it works on gcc 4.x now. I would never have found this without Vince, thanks so much

git-svn-id: svn+ssh://scm.gforge.inria.fr/svn/simgrid/simgrid/trunk@2489 48e7efb5-ca39-0410-a469-dd3cf9ba447f