qemu: Add extra checks for secret destroy API's

Remove the possibility that a NULL hostdev->privateData or a
disk->privateData could crash libvirtd by checking for NULL
before dereferencing for the secinfo structure in the
qemuDomainSecret{Disk|Hostdev}Destroy functions. The hostdevPriv
could be NULL if qemuProcessNetworkPrepareDevices adds a new
hostdev during virDomainNetGetActualHostdev that then gets
inserted via virDomainHostdevInsert. The hostdevPriv was added
by commit id '27726d8c' and is currently only used by scsi hostdev.

src/qemu/qemu_domain.c

@@ -903,7 +903,7 @@ qemuDomainSecretDiskDestroy(virDomainDiskDefPtr disk)
 {
     qemuDomainDiskPrivatePtr diskPriv = QEMU_DOMAIN_DISK_PRIVATE(disk);
 
-    if (!diskPriv->secinfo)
+    if (!diskPriv || !diskPriv->secinfo)
         return;
 
     qemuDomainSecretInfoFree(&diskPriv->secinfo);
@@ -964,7 +964,7 @@ qemuDomainSecretHostdevDestroy(virDomainHostdevDefPtr hostdev)
     qemuDomainHostdevPrivatePtr hostdevPriv =
         QEMU_DOMAIN_HOSTDEV_PRIVATE(hostdev);
 
-    if (!hostdevPriv->secinfo)
+    if (!hostdevPriv || !hostdevPriv->secinfo)
         return;
 
     qemuDomainSecretInfoFree(&hostdevPriv->secinfo);