[GHC] #11978: running a profiled build of shake test suite with rts args +RTS -hb -N10 triggers SIGSEGV

[GHC]

#11978: running a profiled build of shake test suite with rts args  +RTS -hb -N10
triggers SIGSEGV
---------------------------------+----------------------------------------
        Reporter:  carter        |                Owner:  erikd
            Type:  bug           |               Status:  patch
        Priority:  highest       |            Milestone:  8.0.2
       Component:  Compiler      |              Version:  8.0.1
      Resolution:                |             Keywords:
Operating System:  MacOS X       |         Architecture:  Unknown/Multiple
 Type of failure:  None/Unknown  |            Test Case:
      Blocked By:                |             Blocking:
 Related Tickets:  #4820         |  Differential Rev(s):  Phab:D2174
       Wiki Page:                |
---------------------------------+----------------------------------------

Comment (by erikd):

 With @bgamari's ok, I've taken ownership of this ticket.

 I have a fix very similar to Ben's (with an extra `ASSERT`) and a test.
 However the test also triggers the bug in #12009 so I need to fix that bug
 before I can commit the fix and test for this one.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11978#comment:21>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler