PrepareMerge type admin message leads to crash

[cfzjywxk]

Applying logic seems to has some problem processing prepare_merge messages.

applier.go:838: [info] [region 8] 9 execute admin command. term 6, index 11, command cmd_type:PrepareMerge prepare_merge:<min_index:11 target:<id:24 start_key:"t\000\000\000\000\000\000\000\370" end_key:"t\200\000\000\000\000\000\000\377\005\000\000\000\000\000\000\000\370" region_epoch:<conf_ver:5 version:6 > peers:<id:25 store_id:1 > peers:<id:26 store_id:12 > peers:<id:27 store_id:13 > > > 
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0xfd4113]

goroutine 120 [running]:
github.com/ngaut/unistore/tikv/raftstore.(*applier).execAdminCmd(0xc011095140, 0xc0001c7f40, 0xc021737cc8, 0x0, 0x0, 0x30, 0x1221f00, 0xc000087b01, 0xc021834000)
	/home/rwork/go/src/github.com/pingcap/unistore/tikv/raftstore/applier.go:869 +0xb3
github.com/ngaut/unistore/tikv/raftstore.(*applier).execRaftCmd(0xc011095140, 0xc0001c7f40, 0xc000087cc8, 0x70, 0x70, 0x12e2c40, 0x0, 0xe, 0x58)
	/home/rwork/go/src/github.com/pingcap/unistore/tikv/raftstore/applier.go:828 +0x9f
github.com/ngaut/unistore/tikv/raftstore.(*applier).applyRaftCmd(0xc011095140, 0xc0001c7f40, 0xb, 0x6, 0xc000087cc8, 0x0, 0x0, 0x0, 0x10)
	/home/rwork/go/src/github.com/pingcap/unistore/tikv/raftstore/applier.go:758 +0xf3
github.com/ngaut/unistore/tikv/raftstore.(*applier).processRaftCmd(0xc011095140, 0xc0001c7f40, 0xb, 0x6, 0xc000087cc8, 0x0, 0x7f84897d4460, 0x0)
	/home/rwork/go/src/github.com/pingcap/unistore/tikv/raftstore/applier.go:729 +0xae
github.com/ngaut/unistore/tikv/raftstore.(*applier).handleRaftEntryNormal(0xc011095140, 0xc0001c7f40, 0xc00029e230, 0x0, 0x1, 0xc021788240)
	/home/rwork/go/src/github.com/pingcap/unistore/tikv/raftstore/applier.go:647 +0x35a
github.com/ngaut/unistore/tikv/raftstore.(*applier).handleRaftCommittedEntries(0xc011095140, 0xc0001c7f40, 0xc00029e230, 0x1, 0x1)
	/home/rwork/go/src/github.com/pingcap/unistore/tikv/raftstore/applier.go:600 +0x49c
github.com/ngaut/unistore/tikv/raftstore.(*applier).handleApply(0xc011095140, 0xc0001c7f40, 0xc0217964b0)
	/home/rwork/go/src/github.com/pingcap/unistore/tikv/raftstore/applier.go:1459 +0x9a
github.com/ngaut/unistore/tikv/raftstore.(*applier).handleTask(0xc011095140, 0xc0001c7f40, 0x12d, 0x8, 0x10e5340, 0xc0217964b0)
	/home/rwork/go/src/github.com/pingcap/unistore/tikv/raftstore/applier.go:1552 +0x71
github.com/ngaut/unistore/tikv/raftstore.(*raftWorker).runApply(0xc000124ea0, 0xc01b76a010)
	/home/rwork/go/src/github.com/pingcap/unistore/tikv/raftstore/peer_state.go:263 +0x147
created by github.com/ngaut/unistore/tikv/raftstore.(*raftWorker).run
	/home/rwork/go/src/github.com/pingcap/unistore/tikv/raftstore/peer_state.go:122 +0x64

Current solution:
Region merge not supported yet, use merge-schedule-limit to disable region merge