bus/peer: fix catching OOM when linking match-rules

When the match-rule infrastructure was extended to have per-key lookup
tables, all call-sites had to be amended to catch errors when linking
new rules. One such site was missed, so fix this and catch the error
properly.

The conversion was initially performed in:

    commit 6c2b082
    Author: Tom Gundersen <teg@jklm.no>
    Date:   Wed Apr 25 17:16:47 2018 +0200

        match: make match_rule_link() fallible

This fixes a possible bug where ENOMEM is ignored and thus a match rule
is silently dropped instead of being linked and retained.

Reported-by: Mark Esler <mark.esler@canonical.com>
Signed-off-by: David Rheinsberg <david@readahead.eu>

src/bus/peer.c

@@ -548,7 +548,9 @@ static int peer_link_match(Peer *peer, MatchRule *rule, bool monitor) {
                 case ADDRESS_TYPE_ID: {
                         sender = peer_registry_find_peer(&peer->bus->peers, addr.id);
                         if (sender) {
-                                match_rule_link(rule, &sender->sender_matches, monitor);
+                                r = match_rule_link(rule, &sender->sender_matches, monitor);
+                                if (r)
+                                        return error_fold(r);
                         } else if (addr.id >= peer->bus->peers.ids) {
                                 /*
                                  * This peer does not yet exist, by the same