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

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 <m.mizuma@jp.fujitsu.com>
Fixes: 2e94002d2ace4e4a6dbfc13a84fdab28f22c5c4a
Reviewed-by: Ján Tomko <jtomko@redhat.com>
Signed-off-by: Ján Tomko <jtomko@redhat.com>