Fix possible panics in slice type assertion

[saschagrunert]

What type of PR is this?

• bug
• cleanup
• documentation
• feature

What this PR does / why we need it:

Before accessing the slices .Value() we should type check that it is
that actual type, otherwise we will end-up in a panic.

Which issue(s) this PR fixes:

None

Special notes for your reviewer:

None

Testing

Reproducer:

package main

import (
	"os"

	"github.com/urfave/cli/v2"
)

func main() {
	app := cli.NewApp()
	app.Flags = []cli.Flag{&cli.StringFlag{Name: "test"}}
	app.Action = func(ctx *cli.Context) error {
		ctx.StringSlice("test")
		return nil
	}
	if err := app.Run(os.Args); err != nil {
		os.Exit(1)
	}
}

Release Notes

- Fixed possible runtime panic in slice parsing

[codecov]

Codecov Report

Merging #1049 into master will increase coverage by 0.23%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #1049      +/-   ##
==========================================
+ Coverage    72.6%   72.83%   +0.23%     
==========================================
  Files          33       33              
  Lines        2482     2474       -8     
==========================================
  Hits         1802     1802              
+ Misses        569      565       -4     
+ Partials      111      107       -4

Impacted Files	Coverage Δ
flag_int64_slice.go	78.33% <100%> (+2.52%)	⬆️
flag_int_slice.go	74.62% <100%> (+2.16%)	⬆️
flag_float64_slice.go	57.14% <100%> (+1.75%)	⬆️
flag_string_slice.go	79.66% <100%> (+2.61%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update f1a114a...ca666ae. Read the comment docs.

[rliebz]

Good catch, thanks for this!

[asahasrabuddhe]

Looks good to me 👍