adding locking to avoid nil VolumeLocationList
fix panic: runtime error: invalid memory address or nil pointer dereference
Oct 22 00:53:44 bedb-master1 weed[8055]: [signal SIGSEGV: segmentation violation code=0x1 addr=0x8 pc=0x17658da]
Oct 22 00:53:44 bedb-master1 weed[8055]: goroutine 310 [running]:
Oct 22 00:53:44 bedb-master1 weed[8055]: github.com/chrislusf/seaweedfs/weed/topology.(*VolumeLocationList).Length(...)
Oct 22 00:53:44 bedb-master1 weed[8055]: #011/root/seaweedfs/weed/topology/volume_location_list.go:35
Oct 22 00:53:44 bedb-master1 weed[8055]: github.com/chrislusf/seaweedfs/weed/topology.(*VolumeLayout).enoughCopies(...)
Oct 22 00:53:44 bedb-master1 weed[8055]: #011/root/seaweedfs/weed/topology/volume_layout.go:376
Oct 22 00:53:44 bedb-master1 weed[8055]: github.com/chrislusf/seaweedfs/weed/topology.(*VolumeLayout).ensureCorrectWritables(0xc000111d50, 0xc000b55438)
Oct 22 00:53:44 bedb-master1 weed[8055]: #011/root/seaweedfs/weed/topology/volume_layout.go:202 +0x5a
Oct 22 00:53:44 bedb-master1 weed[8055]: github.com/chrislusf/seaweedfs/weed/topology.(*Topology).SyncDataNodeRegistration(0xc00042ac60, 0xc001454d30, 0x1, 0x1, 0xc0005fc000, 0xc00135de40, 0x4, 0xc00135de50, 0x10, 0x10d, ...)
Oct 22 00:53:44 bedb-master1 weed[8055]: #011/root/seaweedfs/weed/topology/topology.go:224 +0x616
Oct 22 00:53:44 bedb-master1 weed[8055]: github.com/chrislusf/seaweedfs/weed/server.(*MasterServer).SendHeartbeat(0xc000162700, 0x23b97c0, 0xc000ae2c90, 0x0, 0x0)
Oct 22 00:53:44 bedb-master1 weed[8055]: #011/root/seaweedfs/weed/server/master_grpc_server.go:106 +0x325
Oct 22 00:53:44 bedb-master1 weed[8055]: github.com/chrislusf/seaweedfs/weed/pb/master_pb._Seaweed_SendHeartbeat_Handler(0x1f8e7c0, 0xc000162700, 0x23b0a60, 0xc00024b440, 0x3172c38, 0xc000ab7100)
Oct 22 00:53:44 bedb-master1 weed[8055]: #011/root/seaweedfs/weed/pb/master_pb/master.pb.go:4250 +0xad
Oct 22 00:53:44 bedb-master1 weed[8055]: google.golang.org/grpc.(*Server).processStreamingRPC(0xc0001f31e0, 0x23bb800, 0xc000ac5500, 0xc000ab7100, 0xc0001fea80, 0x311fec0, 0x0, 0x0, 0x0)
Oct 22 00:53:44 bedb-master1 weed[8055]: #011/root/go/pkg/mod/google.golang.org/grpc@v1.29.1/server.go:1329 +0xcd8
Oct 22 00:53:44 bedb-master1 weed[8055]: google.golang.org/grpc.(*Server).handleStream(0xc0001f31e0, 0x23bb800, 0xc000ac5500, 0xc000ab7100, 0x0)
Oct 22 00:53:44 bedb-master1 weed[8055]: #011/root/go/pkg/mod/google.golang.org/grpc@v1.29.1/server.go:1409 +0xc5c
Oct 22 00:53:44 bedb-master1 weed[8055]: google.golang.org/grpc.(*Server).serveStreams.func1.1(0xc0001ce8b0, 0xc0001f31e0, 0x23bb800, 0xc000ac5500, 0xc000ab7100)
Oct 22 00:53:44 bedb-master1 weed[8055]: #011/root/go/pkg/mod/google.golang.org/grpc@v1.29.1/server.go:746 +0xa5
Oct 22 00:53:44 bedb-master1 weed[8055]: created by google.golang.org/grpc.(*Server).serveStreams.func1
Oct 22 00:53:44 bedb-master1 weed[8055]: #011/root/go/pkg/mod/google.golang.org/grpc@v1.29.1/server.go:744 +0xa5
Oct 22 00:53:44 bedb-master1 systemd[1]: weedmaster.service: Main process exited, code=exited, status=2/INVALIDARGUMENT
Oct 22 00:53:44 bedb-master1 systemd[1]: weedmaster.service: Failed with result 'exit-code'.