Don't try to close the client config file if opening fails

$ q
panic: runtime error: invalid memory address or nil pointer dereference
[signal 0xb code=0x1 addr=0x0 pc=0x9c124]

goroutine 1 [running]:
os.(*File).Close(0x0, 0xf800000010, 0x0, 0xf840054e40)
/usr/local/go/src/pkg/os/file_unix.go:99 +0x1c
github.com/miekg/dns.ClientConfigFromFile(0x1c3294, 0x6374652f00000010, 0x0, 0xf840054e40, 0xf840054de0, ...)
/Users/ask/go/src/github.com/miekg/dns/clientconfig.go:30 +0xcc
main.main()
/Users/ask/go/src/github.com/miekg/dns/ex/q/q.go:50 +0x45b
Work without resolv.conf if the nameserver is specified with @...
Merge pull request #40 from abh/q-fixes

Make `q` work better if resolv.conf is missing