bml/r2: fix a problem identified by Gilles

This commit fixes two issues:

 - This intent of the code @ bml_r2.c:486 is to prevent calling the
   btl_del_procs more than once for a given proc. Gilles correctly
   identified there was a problem in this code but r31786 we not the
   correct fix.

 - Fix a segmentation fault in r2 finalize revealed by the fact we
   actually call del_procs now.

cmr=v1.8.2:reviewer=ggouaillardet:ticket=#4645