add debug check for mempool double free (or misuse of BLI_MEMPOOL_ALLOW_ITER),
some reports in the tracker show errors on bmesh iteration which could be caused by this.


git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@54569 954f8c5b-7b00-dc11-b283-0030488c597c