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.
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