Protect against NULL pointer flaws in monitor usage

History has shown that there are frequent bugs in the QEMU driver
code leading to the monitor being invoked with a NULL pointer.
Although the QEMU driver code should always report an error in
this case before invoking the monitor, as a safety net put in a
generic check in the monitor code entry points.

* src/qemu/qemu_monitor.c: Safety net to check for NULL monitor
  object