coap-server.c: Immediate exit when no endpoints defined

get_context() creates a new context object and adds coap_endpoint_t
objects for all listen addresses. The function now clears the
context object and returns NULL when no endpoint was created,
causing the server to exit immediately.

examples/coap-server.c

@@ -1119,6 +1119,8 @@ get_context(const char *node, const char *port) {
   }
 
   fprintf(stderr, "no context available for interface '%s'\n", node);
+  coap_free_context(ctx);
+  ctx = NULL;
 
 finish:
   freeaddrinfo(result);