FIX #2327: a fault in the thunk-selector machinery (again)

This program contains an expression of the form

   let x = snd (_, snd (_, snd (_, x)))

(probably not explicitly, but that's what appears in the heap at
runtime).  Obviously the program should deadlock if it ever enters
this thing, but apparently the test program in #2327 never does.

The GC tries to evaluate the snd closures, and gets confused due to
the loop.  In particular the earlier fix for #1038 was to blame.