R12B3 tracing crashes emulator

[Simon Cornish]

Tracing on [all], or specifically [running, scheduler_id] trashes the
stack leading to some sort of SEGV or BUS fault. Seen on Solaris
i386/64-bit and OS/X i386/32-bit.
This doesn't seem to be related to the other thing reported/fixed
recently in R13.
Patch attached.
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