cli: fix download segfaults when outputs directive is wrong

[tiborsimko]

Current behaviour

reana-client-go segfaults when the outputs directive is wrong.

For example, remove it:

$ cd reana-demo-helloworld
$ vim reana.yaml
$ git diff
diff --git a/reana.yaml b/reana.yaml
index 0fc4418..2d1eb36 100644
--- a/reana.yaml
+++ b/reana.yaml
@@ -18,6 +18,3 @@ workflow:
               --inputfile "${inputfile}"
               --outputfile "${outputfile}"
               --sleeptime ${sleeptime}
-outputs:
-  files:
-   - results/greetings.txt

Run such a workflow:

$ reana-dev run-example -c r-d-helloworld -w serial -b kubernetes --submit-only

Try to download all outputs:

$ rcg download -w helloworld-serial-kubernetes.3
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x20 pc=0xb057b6]

goroutine 1 [running]:
reanahub/reana-client-go/cmd.(*downloadOptions).run(0xc0003ae390, 0x0?, {0xc0002c3580?, 0x0?, 0x0?})
        .../reana-client-go/cmd/download.go:98 +0x76
reanahub/reana-client-go/cmd.newDownloadCmd.func1(0xc0003b0a00?, {0xc0002c3580?, 0x2?, 0x2?})
        .../reana-client-go/cmd/download.go:63 +0x31
github.com/spf13/cobra.(*Command).execute(0xc0003b0a00, {0xc0002c3560, 0x2, 0x2})
        /home/tibor/private/go/pkg/mod/github.com/spf13/[email protected]/command.go:872 +0x694
github.com/spf13/cobra.(*Command).ExecuteC(0xc0003a6280)
        /home/tibor/private/go/pkg/mod/github.com/spf13/[email protected]/command.go:990 +0x3bd
github.com/spf13/cobra.(*Command).Execute(0xc0000061a0?)
        /home/tibor/private/go/pkg/mod/github.com/spf13/[email protected]/command.go:918 +0x19
main.main()
        .../reana-client-go/main.go:28 +0x25

Expected behaviour

The reana-client-go should not segfault but emit an output message to the user that there is nothing to download and explain why.