resolve: mdns_packet_extract_matching_rrs() may return 0
Fixes the following assertion:
---
Assertion 'r > 0' failed at src/resolve/resolved-mdns.c:180, function mdns_do_tiebreak(). Aborting.
---
(cherry picked from commit f2605af1f2e770818bbc6bad2561acdbd25a38ad)