qemu: monitor: Fix device matching in qemuMonitorJSONBlockIoThrottleInfo

We should compare the alias/qdev id only when it was provided by the
caller and when it was found in the reply. Otherwise we could
dereference a NULL pointer. STRNEQ_NULLABLE is not appropriate since
it would return 'true' if the string was not present in the JSON output.

Found by Coverity.
Signed-off-by: NPeter Krempa <pkrempa@redhat.com>
Reviewed-by: NJohn Ferlan <jferlan@redhat.com>

src/qemu/qemu_monitor_json.c

@@ -5001,8 +5001,8 @@ qemuMonitorJSONBlockIoThrottleInfo(virJSONValuePtr io_throttle,
             goto cleanup;
         }
 
-        if ((drivealias && STRNEQ(current_drive, drivealias)) ||
-            (qdevid && STRNEQ(current_qdev, qdevid)))
+        if ((drivealias && current_drive && STRNEQ(current_drive, drivealias)) ||
+            (qdevid && current_qdev && STRNEQ(current_qdev, qdevid)))
             continue;
 
         found = true;