[GHC] #9068: Don't uninstall signal handlers if none were installed

[GHC]

#9068: Don't uninstall signal handlers if none were installed
--------------------------+------------------------------------------------
       Reporter:  tomgr   |             Owner:  simonmar
           Type:  bug     |            Status:  new
       Priority:  normal  |         Milestone:
      Component:          |           Version:  7.8.2
  Runtime System          |  Operating System:  Windows
       Keywords:          |   Type of failure:  Incorrect result at runtime
   Architecture:          |         Test Case:
  Unknown/Multiple        |          Blocking:
     Difficulty:          |
  Unknown                 |
     Blocked By:          |
Related Tickets:          |
--------------------------+------------------------------------------------
 GHC 7.8.2 calls resetSignalHandlers even when --install-signal-handlers=no
 is specified. On windows this results in the call to SetConsoleCtrlHandler
 failing.

 The attached patch only calls resetSignalHandlers when there are signal
 handlers to remove (it mirrors the logic used when installing the
 handlers).

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9068>
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