cmd/zlint: fix panic w/ deref of nil registry. (#385)
cmd/zlint: fix panic w/ deref of nil registry. (#385)

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

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

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
```
zmap/zlint,zmap/zlint