copy-structure broken with dx structs
copy-structure (and thus the default copied from defstruct) uses %instance-length to determine the length of the structure to copy.
But %instance-length is rounded up to a multiple of 2-minus-1 by dd-instance-length so that alignment works right, so some structs have an extra word of padding on the end.
DX-allocated structs don't pre-initialize the memory used, and so the padding word is filled with garbage. Garbage on the stack is okay (at least on x86/x86-64), so no problem.
But then, copy-structure copies the word of garbage to the heap. And next time there's a GC...BOOM.
I've attached the obvious patch to copy-structure.