Fix 32bit external pointer.

For 32bit external pointer it was performing a divide by size, and for
things not managed by snmalloc this was causing a crash.  This checks
for zero, and gives the start of the address range as the start of the
object.

Author: mjp41

src/mem/sizeclasstable.h

@@ -357,6 +357,8 @@ namespace snmalloc
     }
     else
     {
+      if (size == 0)
+        return 0;
       return slab_start + (offset / size) * size;
     }
   }