Open onu adapter crashes when processing requests and device is deleted

Open onu adapter will crash when the device is deleted during processing of request, where the ONU data is being stored in KV store. The Onu Indication object from the device handler will be invalid because the device is deleted and at the same if the onu data is attempted to be stored in KV store, it will lead to crash.

panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x4 pc=0x1109544]

goroutine 317493 [running]:
github.com/opencord/voltha-openonu-adapter-go/internal/pkg/mib.(*OnuDeviceEntry).storeDataInOnuKvStore(0xc07ec3dc00, 0x1692780, 0xc0912365d0, 0xc001001b01)
        /go/src/internal/pkg/mib/onu_device_entry.go:640 +0xa4
created by github.com/opencord/voltha-openonu-adapter-go/internal/pkg/mib.(*OnuDeviceEntry).UpdateOnuKvStore
        /go/src/internal/pkg/mib/onu_device_entry.go:625 +0x9