fdc: fix segfault in fdctrl_stop_transfer() when DMA is disabled

Commit c8a35f1 "fdc: use IsaDma interface instead of global DMA_*
functions" accidentally introduced a segfault in fdctrl_stop_transfer() for
non-DMA transfers.

If fdctrl->dma_chann has not been configured then the fdctrl->dma interface
reference isn't initialised during isabus_fdc_realize(). Unfortunately
fdctrl_stop_transfer() unconditionally references the DMA interface when
finishing the transfer causing a NULL pointer dereference.

Fix the issue by adding a check in fdctrl_stop_transfer() so that the DMA
interface reference and release method is only invoked if fdctrl->dma_chann
has been set.

(This issue was discovered by Martin testing a recent change in the NetBSD
installer under qemu-system-sparc)

Cc: qemu-stable@nongnu.org
Reported-by: Martin Husemann <martin@duskware.de>
Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Hervé Poussineau <hpoussin@reactos.org>
Reviewed-by: John Snow <jsnow@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
(cherry picked from commit 441f669)
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>

hw/block/fdc.c

@@ -1617,7 +1617,7 @@ static void fdctrl_stop_transfer(FDCtrl *fdctrl, uint8_t status0,
     fdctrl->fifo[5] = cur_drv->sect;
     fdctrl->fifo[6] = FD_SECTOR_SC;
     fdctrl->data_dir = FD_DIR_READ;
-    if (!(fdctrl->msr & FD_MSR_NONDMA)) {
+    if (fdctrl->dma_chann != -1 && !(fdctrl->msr & FD_MSR_NONDMA)) {
         IsaDmaClass *k = ISADMA_GET_CLASS(fdctrl->dma);
         k->release_DREQ(fdctrl->dma, fdctrl->dma_chann);
     }