Fix Coverity Scan issue 1212267 -- invalid free of array.

The code was correct but contained a typo involving two similarly-named fields: 
  mark_space -- Holds pre-allocated space.
  mark_stack -- Points to pre-allocated or dynamically allocated space.
mark_stack may be freed, but only if it is not set equal to mark_space.  
mark_space should never be freed, since it is owned by the containing qio_channel_t
object.

[tested on io/ (std)]
[not reviewed (trivial)]


git-svn-id: http://svn.code.sf.net/p/chapel/code/trunk@23631 3a8e244f-b0f2-452b-bcba-4c88e055c3ca