check in stderrFrame is nil before logging stderrFrame.Data

[kmulvey]

Prevent panic if stderrFrame is nil

debug logging:

logUI: &{reader:0xc000014010 writer:0xc000014018 errorWriter:0xc000014020 underlyingUI:0xc000a26750 isColor:true outputColor:{Code:0 Bold:false} infoColor:{Code:92 Bold:false} errorColor:{Code:91 Bold:false} warnColor:{Code:93 Bold:false}} 
stderrFrame: <nil> 



panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x8 pc=0x1fbf565]

goroutine 1 [running]:
github.com/hashicorp/nomad/command.(*AllocLogsCommand).tailMultipleFiles(0xc00040d7a0, 0xc000a2ae70, 0x24?)
	/home/kmulvey/src/go/src/github.com/hashicorp/nomad/command/alloc_logs.go:403 +0x645
github.com/hashicorp/nomad/command.(*AllocLogsCommand).Run(0xc00040d7a0, {0xc0000520c0, 0x3, 0x3})
	/home/kmulvey/src/go/src/github.com/hashicorp/nomad/command/alloc_logs.go:233 +0x967
github.com/mitchellh/cli.(*CLI).Run(0xc000a07cc0)
	/home/kmulvey/src/go/pkg/mod/github.com/mitchellh/[email protected]/cli.go:262 +0x5f8
main.Run({0xc0000520b0, 0x4, 0x4})
	/home/kmulvey/src/go/src/github.com/hashicorp/nomad/main.go:107 +0x28a
main.main()
	/home/kmulvey/src/go/src/github.com/hashicorp/nomad/main.go:77 +0x4e

[hashicorp-cla]

All committers have signed the CLA.

[jrasell]

LGTM and thanks @kmulvey! Would you mind adding a changelog entry; this can be setup via the make cl command within the Nomad repository root.