discoverd/client: Don’t return heartbeater if error is encountered
discoverd/client: Don’t return heartbeater if error is encountered

This avoids mistakes where the heartbeater is assigned somewhere and
later checked if nil.

Signed-off-by: Jonathan Rudenberg <3692bfa45759a67d83aedf0045f6cb635a966abf@titanous.com>
lmars/flynn,lmars/flynn,philiplb/flynn,lmars/flynn,benbjohnson/flynn,benbjohnson/flynn,flynn/flynn,lmars/flynn,philiplb/flynn,benbjohnson/flynn,benbjohnson/flynn,philiplb/flynn,philiplb/flynn,flynn/flynn,flynn/flynn,philiplb/flynn,flynn/flynn,flynn/flynn,lmars/flynn,benbjohnson/flynn,benbjohnson/flynn