Fix segfault when node no longer connected

Summary:
nodestate can be NULL when node is no longer connected,
leading to a segmentation fault (refer T64).

Added assertions to other places where node state was
obtained and later used without checking if non-null.

Test Plan: make check

Reviewers: #bitcoin_abc, deadalnix, sickpig, awemany, dagurval, CCulianu

Reviewed By: #bitcoin_abc, deadalnix, dagurval

Differential Revision: https://reviews.bitcoinabc.org/D324