qemu: check for json allocation failure

Detected by Coverity.  Introduced in commit b1b5b51a.

* src/qemu/qemu_monitor_json.c (qemuMonitorJSONGetBlockInfo):
Avoid null dereference.

src/qemu/qemu_monitor_json.c

@@ -1364,6 +1364,9 @@ int qemuMonitorJSONGetBlockInfo(qemuMonitorPtr mon,
     virJSONValuePtr reply = NULL;
     virJSONValuePtr devices;
 
+    if (!cmd)
+        return -1;
+
     ret = qemuMonitorJSONCommand(mon, cmd, &reply);
     if (ret == 0)
         ret = qemuMonitorJSONCheckError(cmd, reply);