When interpreter is not profiled (see `interpreterProfiled` in
`DynFlags`) bytecode generator generates a NULL pointer as the cost
centre of a `BRK_FUN` instruction:

    let cc | interpreterProfiled dflags = cc_arr ! tick_no
           | otherwise = toRemotePtr nullPtr
    let breakInstr = BRK_FUN (fromIntegral tick_no) (getUnique this_mod) cc
    return $ breakInstr `consOL` code

We now take this into account when disassembling `BRK_FUN`.

Reviewers: bgamari, simonmar, erikd

Subscribers: rwbarton, thomie, carter

Differential Revision: https://phabricator.haskell.org/D4490