rust_new_task_in_sched might fail to acquire scheduler

[graydon]

Is this a real condition? There is a fixme on it. Clarify.

[bblum]

Looks possible at the interface level. Currently this builtin is only used by test/run-pass/rt-sched-1.rs (testcase) and by libcore/task.rs (within spawn_raw). At both callsites the sched_id is ensured to be right, but a user might well crash the runtime by calling the builtin without going through the task library.

Easiest "fix" would be to say the return value is maybe-null, return prematurely "if we didn't get the sched", add some nonnull asserts around the current callsites in rust code.