Avoid panic for missing flag value

[rliebz]

Currently, in cases where a flag value is required but not passed, and short option handling is enabled, a panic will occur due to a nil pointer dereference. This prevents that situation from occurring, instead propagating the appropriate flag-parsing error.

Small example to reproduce issue:

package main

import (
	"os"

	"github.com/urfave/cli"
)

func main() {
	app := cli.NewApp()
	app.UseShortOptionHandling = true
	app.Flags = []cli.Flag{
		cli.StringFlag{Name: "flag"},
	}
	app.Run(os.Args)
}

Before:

$ ./simple-app --flag
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x20 pc=0x10d5d42]
...

After:

$ ./simple-app --flag
Incorrect Usage. flag needs an argument: -flag

NAME:
   simple-app - A new cli application

USAGE:
   simple-app [global options] command [command options] [arguments...]

VERSION:
   0.0.0

COMMANDS:
   help, h  Shows a list of commands or help for one command

GLOBAL OPTIONS:
   --flag value
   --help, -h     show help
   --version, -v  print the version

[codecov]

Codecov Report

Merging #893 into master will increase coverage by 0.04%.
The diff coverage is 76.47%.

@@            Coverage Diff             @@
##           master     #893      +/-   ##
==========================================
+ Coverage   71.41%   71.45%   +0.04%     
==========================================
  Files          30       30              
  Lines        2389     2393       +4     
==========================================
+ Hits         1706     1710       +4     
  Misses        577      577              
  Partials      106      106

Impacted Files	Coverage Δ
app.go	84.45% <100%> (ø)	⬆️
command.go	82.58% <50%> (-0.98%)	⬇️
parse.go	94.73% <77.77%> (+5.54%)	⬆️

---

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[saschagrunert]

LGTM