m68k: mac: Don't send uninitialized data in IOP message reply

Clear the message reply before calling iop_complete(). This code path is
not normally executed but should that happen let's arrange for consistent
behaviour from the IOP.
Signed-off-by: Finn Thain <fthain@telegraphics.com.au>
Tested-by: Stan Johnson <userm57@yahoo.com>
Cc: Joshua Thompson <funaho@jurai.org>
Link: https://lore.kernel.org/r/8e35df4d193b082cb6285b1f30c949ff7e30e99e.1590880333.git.fthain@telegraphics.com.auSigned-off-by: Geert Uytterhoeven <geert@linux-m68k.org>

arch/m68k/mac/iop.c

@@ -449,6 +449,7 @@ static void iop_handle_recv(uint iop_num, uint chan)
 		iop_pr_debug("unclaimed message on iop_num %d chan %d\n",
 		             iop_num, chan);
 		iop_pr_debug("%*ph\n", IOP_MSG_LEN, msg->message);
+		memset(msg->reply, 0, IOP_MSG_LEN);
 		iop_complete_message(msg);
 	}
 }