qemu: domain: Tolerate NULL @disk in qemuDomainPrepareDiskSourceData

In some cases we want to prepare a @src which is not meant to belong to
a disk and thus does not require us to copy the data. Allow passing in
NULL @disk into qemuDomainPrepareDiskSourceData.
Signed-off-by: NPeter Krempa <pkrempa@redhat.com>
Reviewed-by: NDaniel Henrique Barboza <danielhb413@gmail.com>

src/qemu/qemu_domain.c

@@ -15082,6 +15082,9 @@ void
 qemuDomainPrepareDiskSourceData(virDomainDiskDefPtr disk,
                                 virStorageSourcePtr src)
 {
+    if (!disk)
+        return;
+
     /* transfer properties valid only for the top level image */
     if (src == disk->src)
         src->detect_zeroes = disk->detect_zeroes;