remove zero value balances from snapshot

[paulhandy]

[Pomyk]

With this patch I get error after about 15 minutes from starting a fresh node: 02/06 10:51:22.021 [Solid Milestone Tracker] ERROR com.iota.iri.Milestone - Error during Solid Milestone updating java.lang.NullPointerException: null at com.iota.iri.Snapshot.lambda$null$1(Snapshot.java:121) ~[iri.jar:na] at java.util.HashMap.compute(HashMap.java:1196) ~[na:1.8.0_141] at com.iota.iri.Snapshot.lambda$apply$2(Snapshot.java:117) ~[iri.jar:na] at java.util.HashMap.forEach(HashMap.java:1288) ~[na:1.8.0_141] at com.iota.iri.Snapshot.apply(Snapshot.java:117) ~[iri.jar:na] at com.iota.iri.LedgerValidator.updateSnapshot(LedgerValidator.java:242) ~[iri.jar:na] at com.iota.iri.Milestone.updateLatestSolidSubtangleMilestone(Milestone.java:228) ~[iri.jar:na] at com.iota.iri.Milestone.lambda$init$1(Milestone.java:150) ~[iri.jar:na] at java.lang.Thread.run(Thread.java:748) ~[na:1.8.0_141]

02/06 10:51:22.157 [Solid Milestone Tracker] INFO com.iota.iri.Snapshot - Skipping negative value for address: LTFKDBLKGJMYBLAGZTM9YVNYTVTDTUVRZLYCEEUR9ZNHDYIMAS9WZGQ9QKSZC9DAZPZS9IZCOVOEVEWSC: -7700