virsh: do not unlink NULL file

virsh: do not unlink NULL file

error:could not take a screenshot of xp
==6216== Syscall param unlink(pathname) points to unaddressable byte(s)
==6216==    at 0x373A0D4937: unlink (syscall-template.S:82)
==6216==    by 0x40FD73: cmdScreenshot (virsh.c:3070)
==6216==    by 0x42BA0D: vshCommandRun (virsh.c:14920)
==6216==    by 0x42EC97: main (virsh.c:16379)
==6216==  Address 0x0 is not stack'd, malloc'd or (recently) free'd
==6216==
error:Requested operation is not valid: domain is not running