scm_done_malloc negatives [1.6]

[Kevin Ryde]

In the 1.6 branch, scm_done_malloc has a bit of code,

  if (size < 0) {
    if (scm_mallocated < size)
      /* The byte count of allocated objects has underflowed.  This is
         probably because you forgot to report the sizes of objects you
         have allocated, by calling scm_done_malloc or some such.  When
         the GC freed them, it subtracted their size from
         scm_mallocated, which underflowed.  */
      abort ();


This looks slightly suspicious, since scm_mallocated is a ulong.  I
wonder if it's meant to be "-size" in the comparison?