esp: ensure that async_len is reset to 0 during esp_hard_reset()

If a reset command is sent after data has been transferred into the SCSI buffer
ensure that async_len is reset to 0. Otherwise a subsequent TI command assumes
the SCSI buffer contains data to be transferred to the device causing it to
dereference the stale async_buf pointer.

Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Fixes: https://gitlab.com/qemu-project/qemu/-/issues/724
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20211118100327.29061-2-mark.cave-ayland@ilande.co.uk>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

hw/scsi/esp.c

@@ -894,6 +894,7 @@ void esp_hard_reset(ESPState *s)
     memset(s->wregs, 0, ESP_REGS);
     s->tchi_written = 0;
     s->ti_size = 0;
+    s->async_len = 0;
     fifo8_reset(&s->fifo);
     fifo8_reset(&s->cmdfifo);
     s->dma = 0;