Prevention: never die from extend_info_from_node() failure.

Prevention: never die from extend_info_from_node() failure.

Bug 21242 occurred because we asserted that extend_info_from_node()
had succeeded...even though we already had the code to handle such a
failure.  We fixed that in 93b39c51629ed0ded2bf807cb6.

But there were four other cases in our code where we called
extend_info_from_node() and either tor_assert()ed that it returned
non-NULL, or [in one case] silently assumed that it returned
non-NULL. That's not such a great idea.  This patch makes those
cases check for a bug of this kind instead.

Fixes bug 21372; bugfix on 0.2.3.1-alpha when
extend_info_from_node() was introduced.