spapr.c: check bus != NULL in spapr_get_fw_dev_path()

spapr.c: check bus != NULL in spapr_get_fw_dev_path()

spapr_get_fw_dev_path() is an impl of
FWPathProviderClass::get_dev_path(). This interface is used by
hw/core/qdev-fw.c via fw_path_provider_try_get_dev_path() in two
functions:

- static char *qdev_get_fw_dev_path_from_handler(), which is used only in
qdev_get_fw_dev_path_helper() and it's guarded by "if (dev &&
dev->parent_bus)";

- char *qdev_get_own_fw_dev_path_from_handler(), which is used in
softmmu/bootdevice.c in get_boot_device_path() like this:

    if (dev) {
        d = qdev_get_own_fw_dev_path_from_handler(dev->parent_bus, dev);

This means that, when called via softmmu/bootdevice.c, there's no check
of 'dev->parent_bus' being not NULL. The result is that the "BusState
*bus" arg of spapr_get_fw_dev_path() can potentially be NULL and if, at
the same time, "SCSIDevice *d" is not NULL, we'll hit this line:

    void *spapr = CAST(void, bus->parent, "spapr-vscsi");

And we'll SIGINT because 'bus' is NULL and we're accessing bus->parent.

Adding a simple 'bus != NULL' check to guard the instances where we
access 'bus->parent' can avoid this altogether.

Signed-off-by: Daniel Henrique Barboza <danielhb413@gmail.com>
Reviewed-by: Cédric Le Goater <clg@kaod.org>
Message-Id: <20220121213852.30243-1-danielhb413@gmail.com>
Signed-off-by: Cédric Le Goater <clg@kaod.org>