Fixed bug #531.

git-svn-id: svn://svn.code.sf.net/p/chibios/svn/trunk@7273 35acf78f-673a-0410-8e92-d51de3d6d3f4
[CPP wrappers] MailBox minor code cleanup.

git-svn-id: svn://svn.code.sf.net/p/chibios/svn/trunk@7274 35acf78f-673a-0410-8e92-d51de3d6d3f4