patch applied (ghc): FIX #767 (withMVar family have a bug)

[Simon Marlow]

Fri May 18 05:25:05 PDT 2007  Simon Marlow <simonmar at microsoft.com>
  * FIX #767 (withMVar family have a bug)
  We never want to raise a StackOverflow exception inside
  Control.Exception.block, because the user has no reasonable way of
  handling it, and it invalidates some useful guarantees.

    M ./rts/Schedule.c -1 +6