qemu: avoid null pointer dereference

qemu: avoid null pointer dereference

This code has had problems historically.  As originally
written, in commit 6bcf2501 (Jun 08), it could call unlink
on a random string, nuking an unrelated file.

Then commit 182a80b9 (Sep 09), the code was rewritten to
allocate tmp, with both a use-after-free bug and a chance to
call unlink(NULL).

Commit e206946 (Mar 11) fixed the use-after-free, but not the
NULL dereference.  Thanks to clang for catching this!

* src/qemu/qemu_driver.c (qemudDomainMemoryPeek): Don't call
unlink on NULL.