Improve code fiability.

This commit protects function calls in
openair2/UTIL/MEM/mem_block.c with a mutex so that
many threads calling them at the same time don't mess up.

The mechanism may be a bit overkill, it might be refined
in the future.


git-svn-id: http://svn.eurecom.fr/openair4G/trunk@7788 818b1a75-f10b-46b9-bf7c-635c3b92a50f