mod -O can produce invalid graph

[glennhickey]

To reproduce:

wget http://public.gi.ucsc.edu/~hickey/chrOther.clip.gfa.gaffixed.vg
vg validate chrOther.clip.gfa.gaffixed.vg
graph: valid

vg mod -O ./chrOther.clip.gfa.gaffixed.vg | vg validate -
graph invalid: missing edge between 53612th step (296702:1) and 53613th step (296702:1) of path HG01175.2.JAHALZ010000163.1[3271196-3763370]
graph invalid: missing edge between 53613th step (296702:0) and 53612th step (296702:1) of path HG01175.2.JAHALZ010000163.1[3271196-3763370]
graph invalid: missing edge between 53613th step (296702:1) and 53614th step (296702:1) of path HG01175.2.JAHALZ010000163.1[3271196-3763370]
graph invalid: missing edge between 53614th step (296702:0) and 53613th step (296702:1) of path HG01175.2.JAHALZ010000163.1[3271196-3763370]
graph invalid: missing edge between 53614th step (296702:1) and 53615th step (296702:1) of path HG01175.2.JAHALZ010000163.1[3271196-3763370]
graph invalid: missing edge between 53615th step (296702:0) and 53614th step (296702:1) of path HG01175.2.JAHALZ010000163.1[3271196-3763370]
(etc)