conf: Fix crash in virDomainDefCompatibleDevice

Commit id 'edae027c' blindly assumed that the passed @oldDev
parameter would not be NULL when calling virDomainDeviceGetInfo;
however, commit id 'b6a264e8' passed NULL for AttachDevice
callers under the premise that there wouldn't be a device
to check/update against.
Signed-off-by: NJohn Ferlan <jferlan@redhat.com>

src/conf/domain_conf.c

@@ -27417,9 +27417,12 @@ virDomainDefCompatibleDevice(virDomainDefPtr def,
 {
     virDomainCompatibleDeviceData data = {
         .newInfo = virDomainDeviceGetInfo(dev),
-        .oldInfo = virDomainDeviceGetInfo(oldDev),
+        .oldInfo = NULL,
     };
 
+    if (oldDev)
+        data.oldInfo = virDomainDeviceGetInfo(oldDev);
+
     if (!virDomainDefHasUSB(def) &&
         def->os.type != VIR_DOMAIN_OSTYPE_EXE &&
         virDomainDeviceIsUSB(dev)) {