Memory leak on FreeBSD

[alexcrichton]

==48216== Thread 1:
==48216== 64 bytes in 1 blocks are definitely lost in loss record 5 of 14
==48216==    at 0x11512B3: malloc (in /usr/local/lib/valgrind/vgpreload_memcheck-amd64-freebsd.so)
==48216==    by 0x1B2DBD3: pthread_attr_init (in /lib/libthr.so.3)
==48216==    by 0x4CE3FB: rt::thread::imp::create::h3b6fad6b5d84e835FQ8::v0.11.pre (in /usr/home/alex/rust/foo)
==48216==    by 0x4D2D98: rt::thread::Thread$LT$$LP$$RP$$GT$::spawn_stack::hfcbbb056292868bcqM8::v0.11.pre (in /usr/home/alex/rust/foo)
==48216==    by 0x4341FA: task::spawn_opts::he0f99b3db2022eba8ed::v0.11.pre (in /usr/home/alex/rust/foo)
==48216==    by 0x4337FC: task::Ops.rt..Runtime::spawn_sibling::h917e725db76901bcvnd::v0.11.pre (in /usr/home/alex/rust/foo)
==48216==    by 0x467D1F: rt::task::Task::spawn_sibling::hc96b4b88f9ed4956tb8::v0.11.pre (in /usr/home/alex/rust/foo)
==48216==    by 0x467A8A: task::TaskBuilder::spawn::h4532d4daa7ebcd1838B::v0.11.pre (in /usr/home/alex/rust/foo)
==48216==    by 0x467F77: task::spawn::hb9f56f98f560a8fa0aC::v0.11.pre (in /usr/home/alex/rust/foo)
==48216==    by 0x4063E7: main::h56af26de3ee65144Oba::v0.0 (in /usr/home/alex/rust/foo)
==48216==    by 0x434CF2: start::closure.7747 (in /usr/home/alex/rust/foo)
==48216==    by 0x4CF582: rt::task::Task::run::closure.41207 (in /usr/home/alex/rust/foo)

I believe that we need to call pthread_attr_destroy