Fix a really nasty bug in SMP

Fix a really nasty bug in SMP
In SMP mode a THUNK can change to an IND at any time.  The generic
apply code (stg_ap_p etc.) examines a closure to determine how to
apply it to its arguments, if it is a THUNK it must enter it first in
order to evaluate it.  The problem was that in order to enter the
THUNK, we were re-reading the info pointer, and possibly ending up
with an IND instead of the original THUNK.  It isn't safe to enter the
IND, because it points to a function (functions are never "entered",
only applied).  Solution: we must not re-read the info pointer.