Off-by-one in SME

[jlowellwofford]

When handling unexpected discoveries, there appears to be an off-by-one error in the SME that eventually tries to advance a chain that has no more mutations. Results in:

08:58:09.303:kraken:DEBUG:StateMutationEngine:123e4567-e89d-12d3-a456-425455440096 found a new path
panic: runtime error: index out of range

goroutine 24 [running]:
github.com/hpc/kraken/core.(*StateMutationEngine).advanceMutation(0xc0001c6000, 0xc0000c73e0, 0x24, 0xc000394b80)
	/home/vagrant/go/src/github.com/hpc/kraken/core/StateMutationEngine.go:730 +0x61a
github.com/hpc/kraken/core.(*StateMutationEngine).handleUnexpected(0xc0001c6000, 0xc0000c73e0, 0x24, 0xc0000c7405, 0x9, 0x9ee0e0, 0xc00041e6e8, 0x185)
	/home/vagrant/go/src/github.com/hpc/kraken/core/StateMutationEngine.go:818 +0xded
github.com/hpc/kraken/core.(*StateMutationEngine).updateMutation(0xc0001c6000, 0xc0000c73e0, 0x24, 0xc0000c7405, 0x9, 0xab7c60, 0xc00038f260, 0x99)
	/home/vagrant/go/src/github.com/hpc/kraken/core/StateMutationEngine.go:878 +0x1175
github.com/hpc/kraken/core.(*StateMutationEngine).handleEvent(0xc0001c6000, 0xb771a0, 0xc0001dca50)
	/home/vagrant/go/src/github.com/hpc/kraken/core/StateMutationEngine.go:971 +0x336
github.com/hpc/kraken/core.(*StateMutationEngine).Run(0xc0001c6000)
	/home/vagrant/go/src/github.com/hpc/kraken/core/StateMutationEngine.go:286 +0x6c8
created by github.com/hpc/kraken/core.(*Kraken).Run
	/home/vagrant/go/src/github.com/hpc/kraken/core/Kraken.go:198 +0x10a
got stream read error on mutation stream: rpc error: code = Unavailable desc = transport is closing
got stream read error on mutation stream: rpc error: code = Unavailable desc = transport is closing
got stream send error on logger stream: EOF
got stream send error on logger stream: EOF