cmd/zlint: fix panic w/ deref of nil registry.

[cpu]

This fixes a panic that can occur when there are no filtering arguments provided to the zlint command line tool introduced in 7741587.

This occurs because setLints returned a nil Registry when the intention was to use the global registry:

zlint/cmd/zlint/main.go

Lines 181 to 184 in 7741587

	// If there's no filter options set, use the global registry as-is
	if nameFilter == "" && includeNames == "" && excludeNames == "" && includeSources == "" && excludeSources == "" {
	return nil, nil
	}

Before fix:

$ zlint -list-lints-source
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x38 pc=0x717bd0]

goroutine 1 [running]:
main.main()
	/home/daniel/go/src/github.com/zmap/zlint/cmd/zlint/main.go:85 +0xe0

After fix:

$ zlint -list-lints-source
    AWSLabs
    Apple
    CABF_BR
    CABF_EV
    ETSI_ESI
    Mozilla
    RFC5280
    RFC5480
    RFC5891
    ZLint

[cpu]

Sorry I missed this one 😓