qemuDomainAttachDiskGenericTransient: Add NULL check in case the overlay disk already exists
qemuDomainAttachDiskGenericTransient: Add NULL check in case the overlay disk already exists

When <transient shareBacking='yes'> is set to a disk and the overlay
disk already exists because of something abnormal, libvirt is terminated
by Segmentation fault.

  # virsh start Test0
  error: Disconnected from qemu:///system due to end of file
  error: Failed to start domain 'Test0'
  error: End of file while reading data: Input/output error

Add NULL check for snapdiskdef so that the rollback can work correctly.

Signed-off-by: Masayoshi Mizuma <d5fddbc10d7d0c19347bebdeca1e9c9f92c33d10@jp.fujitsu.com>
Fixes: 2e94002d2ace4e4a6dbfc13a84fdab28f22c5c4a
Reviewed-by: Ján Tomko <4cab11cfb98d3c937327354a78eb07dbb6ee2bc6@redhat.com>
Signed-off-by: Ján Tomko <4cab11cfb98d3c937327354a78eb07dbb6ee2bc6@redhat.com>

olafhering/libvirt,crobinso/libvirt,nertpinx/libvirt,libvirt/libvirt,jfehlig/libvirt,olafhering/libvirt,nertpinx/libvirt,zippy2/libvirt,zippy2/libvirt,nertpinx/libvirt,libvirt/libvirt,crobinso/libvirt,libvirt/libvirt,crobinso/libvirt,zippy2/libvirt,nertpinx/libvirt,olafhering/libvirt,jfehlig/libvirt,olafhering/libvirt,crobinso/libvirt,jfehlig/libvirt,jfehlig/libvirt,zippy2/libvirt,libvirt/libvirt,nertpinx/libvirt