debugnet: fix an errant assertion
We may call debugnet_free() before g_debugnet_pcb_inuse is true,
specifically in the cases where the interface is down or does not
support debugnet. pcb->dp_drv_input is used to hold the real driver
if_input callback while debugnet is in use, so we can check the status
of this field in the assertion.
This can be triggered trivially by trying to configure netdump on an
unsupported interface at the ddb prompt.
Initializing the dp_drv_input field to NULL explicitly is not necessary
but helps display the intent.