only stop networkTopology when it is not nil

[realityone]

Description

The cd.networkTopology is not be checked before calling the Stop method, a nil pointer panic will happen during the peer stop procedure.

panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x20 pc=0x14d2c1b]

goroutine 321 [running]:
panic({0x169f5a0?, 0x2d44570?})
    /usr/local/go/src/runtime/panic.go:1017 +0x3ac fp=0xc000439cc8 sp=0xc000439c18 pc=0x43b2ec
runtime.panicmem(...)
    /usr/local/go/src/runtime/panic.go:261
runtime.sigpanic()
    /usr/local/go/src/runtime/signal_unix.go:861 +0x378 fp=0xc000439d28 sp=0xc000439cc8 pc=0x4526d8
d7y.io/dragonfly/v2/client/daemon.(*clientDaemon).Stop.func1()
    /var/run/dragonfly2-seed-peer/client/daemon/daemon.go:865 +0x57b fp=0xc000439e80 sp=0xc000439d28 pc=0x14d2c1b
sync.(*Once).doSlow(0x9715ac?, 0xc000d924e0?)
    /usr/local/go/src/sync/once.go:74 +0xbf fp=0xc000439ee0 sp=0xc000439e80 pc=0x47d39f
sync.(*Once).Do(...)
    /usr/local/go/src/sync/once.go:65
d7y.io/dragonfly/v2/client/daemon.(*clientDaemon).Stop(0xc000d924e0?)
    /var/run/dragonfly2-seed-peer/client/daemon/daemon.go:816 +0x3b fp=0xc000439f10 sp=0xc000439ee0 pc=0x14d267b
d7y.io/dragonfly/v2/cmd/dfget/cmd.runDaemon.func2()
    /var/run/dragonfly2-seed-peer/cmd/dfget/cmd/daemon.go:198 +0x1c fp=0xc000439f28 sp=0xc000439f10 pc=0x14e1c9c
d7y.io/dragonfly/v2/cmd/dependency.SetupQuitSignalHandler.func1()
    /var/run/dragonfly2-seed-peer/cmd/dependency/dependency.go:152 +0xe8 fp=0xc000439fe0 sp=0xc000439f28 pc=0x13f6188
runtime.goexit()
    /usr/local/go/src/runtime/asm_amd64.s:1650 +0x1 fp=0xc000439fe8 sp=0xc000439fe0 pc=0x471a01
created by d7y.io/dragonfly/v2/cmd/dependency.SetupQuitSignalHandler in goroutine 1
    /var/run/dragonfly2-seed-peer/cmd/dependency/dependency.go:144 +0xd9

Related Issue

Motivation and Context

Screenshots (if appropriate)

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation Update (if none of the other choices apply)

Checklist

• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.

[gaius-qi]

LGTM