fix CVE-2020-25742
patch link: https://lists.nongnu.org/archive/html/qemu-devel/2020-09/msg05294.html
While mapping IRQ level in pci_change_irq_level() routine,
it does not check if pci_get_bus() returned a valid pointer.
It may lead to a NULL pointer dereference issue. Add check to
avoid it.
-> https://ruhr-uni-bochum.sciebo.de/s/NNWP2GfwzYKeKwE?path=%2Flsi_nullptr1
==1183858==Hint: address points to the zero page.
#0 pci_change_irq_level hw/pci/pci.c:259
#1 pci_irq_handler hw/pci/pci.c:1445
#2 pci_set_irq hw/pci/pci.c:1463
#3 lsi_set_irq hw/scsi/lsi53c895a.c:488
#4 lsi_update_irq hw/scsi/lsi53c895a.c:523
#5 lsi_script_scsi_interrupt hw/scsi/lsi53c895a.c:554
#6 lsi_execute_script hw/scsi/lsi53c895a.c:1149
#7 lsi_reg_writeb hw/scsi/lsi53c895a.c:1984
#8 lsi_io_write hw/scsi/lsi53c895a.c:2146
...
Reported-by: Ruhr-University <bugs-syssec@rub.de>
Signed-off-by: Prasad J Pandit <pjp@fedoraproject.org>
