qemu: domain: Tolerate NULL 'cfg' in qemuDomainPrepareDiskSourceChain

The function will be reused in the test code where we don't care much
that the gluster debug level can't be populated from the qemu config.

Set the level only when 'cfg' is passed.
Signed-off-by: NPeter Krempa <pkrempa@redhat.com>
Reviewed-by: NJohn Ferlan <jferlan@redhat.com>

src/qemu/qemu_domain.c

@@ -11871,7 +11871,8 @@ qemuDomainPrepareDiskSourceChain(virDomainDiskDefPtr disk,
     src->detect_zeroes = disk->detect_zeroes;
 
     for (n = src; virStorageSourceIsBacking(n); n = n->backingStore) {
-        if (n->type == VIR_STORAGE_TYPE_NETWORK &&
+        if (cfg &&
+            n->type == VIR_STORAGE_TYPE_NETWORK &&
             n->protocol == VIR_STORAGE_NET_PROTOCOL_GLUSTER &&
             virQEMUCapsGet(qemuCaps, QEMU_CAPS_GLUSTER_DEBUG_LEVEL)) {
             n->debug = true;