fdc: Exit if ISA controller does not support DMA

A "powernv" machine type defines an ISA bus but it does not add any DMA
controller to it so it is possible to hit assert(fdctrl->dma) by
adding "-machine powernv -device isa-fdc".

This replaces assert() with an error message.
Signed-off-by: NAlexey Kardashevskiy <aik@ozlabs.ru>
[thuth: Slightly adjusted error message and updated scripts/device-crash-test]
Signed-off-by: NThomas Huth <thuth@redhat.com>
Message-Id: <1521193892-15552-3-git-send-email-thuth@redhat.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

hw/block/fdc.c

@@ -2695,7 +2695,10 @@ static void isabus_fdc_realize(DeviceState *dev, Error **errp)
     fdctrl->dma_chann = isa->dma;
     if (fdctrl->dma_chann != -1) {
         fdctrl->dma = isa_get_dma(isa_bus_from_device(isadev), isa->dma);
-        assert(fdctrl->dma);
+        if (!fdctrl->dma) {
+            error_setg(errp, "ISA controller does not support DMA");
+            return;
+        }
     }
 
     qdev_set_legacy_instance_id(dev, isa->iobase, 2);