Fix a debug print in disassembler (#14905)

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

(cherry picked from commit 8e341013)

rts/Disassembler.c

@@ -68,7 +68,11 @@ disInstr ( StgBCO *bco, int pc )
       case bci_BRK_FUN:
          debugBelch ("BRK_FUN  " );  printPtr( ptrs[instrs[pc]] );
          debugBelch (" %d ", instrs[pc+1]); printPtr( ptrs[instrs[pc+2]] );
-         debugBelch(" %s\n", ((CostCentre*)(literals[instrs[pc+3]]))->label);
+         CostCentre* cc = (CostCentre*)literals[instrs[pc+3]];
+         if (cc) {
+           debugBelch(" %s", cc->label);
+         }
+         debugBelch("\n");
          pc += 4;
          break;
       case bci_SWIZZLE: