coap_free_endpoint: fix segfault with incorrect -A argument
coap_free_endpoint: fix segfault with incorrect -A argument

`coap-server(1)` segfaults when invoked as:

	$ coap-server -A '2001:db8::1'
	Feb 09 18:28:49 WARN coap_socket_bind_udp: bind: Address not available
	Segmentation fault

In this case, the `ep->context->endpoint` is NULL but
`coap_free_endpoint` still attempts to access it. To prevent this
invocation from causing a segmentation fault, check if
`ep->context->endpoint` is NULL before accessing it.

authmillenon/libcoap,authmillenon/libcoap,authmillenon/libcoap