In print.c, there is a global variable prstack that is not yet scanned.
This causes a crash for comp-tests but this example triggers the same
problem:

(progn
  (defun make-tree (i)
    (cond ((= i 0) nil)
	  (t (list (make-string 10000 i)
		   (make-tree (1- i)) (make-tree (1- i))))))
  (prin1-to-string (make-tree 13)))

We could create an ambiguous root for this but I thought it would be an
interesting exercise to scan it exactly.  It's interesting because this is
a static variable and the type is only declared in print.c.  So I added
a new function igc_xpalloc_exact to igc.h.  That uses a callback that
calls another callback.  I'm not sure if this allowed by the MPS rules
but it seems to work or at least doesn't seem to crash.

WDYT?