Assertion failed: (ref_count == 0), function end_detach on freebsd

[brson]

This showed up in the freebsd bot:

http://bot.rust-lang.org/logs/2012/04/01/2012-05-01T18:11:10Z-668b5492-e641-43c3-84e9-5605e37851fe.html

[catamorphism]

Appears to be fixed by #2523.