MFV c3ccd112: Correct off-by-ones in free_exception of emergency buffer

Note, this has been broken since import in r227825.

PR: https://github.com/pathscale/libcxxrt/issues/29
Reviewed by: emaste (earlier version), kan (informally)
Obtained from: Anton Rang
Relnotes: yes
Sponsored by: EMC / Isilon Storage Division
Differential Revision: https://reviews.freebsd.org/D3733