This has been back-ported to 1.8 and is proposed for 1.7:
r1574868 Don't let invalid mergeinfo stop 'svnadmin dump' from producing a dump. Part of issue #4476 "Mergeinfo containing r0 makes svnsync and svnadmin dump fail". Hi, Philip and Bert in particular, you voted for this to be backported to 1.8 (and it was released in v1.8.9); could you also vote for it for 1.7? - Julian
