Error if terminal is resized while in external command

[ericfreese]

Repro:

• Set up key-binding to run vim
• Run rat and hit key-binding to open vim
• Resize terminal while vim is open
• Exit vim
• See error below

panic: runtime error: makeslice: cap out of range

goroutine 1 [running]:
github.com/ericfreese/rat/lib.(*buffer).StyledLines(0xc420010500, 0x0, 0xfffffffffffff
	/home/ericfreese/golang/src/github.com/ericfreese/rat/lib/buffer.go:83 +0x7d
github.com/ericfreese/rat/lib.(*cmdPager).drawContent(0xc4200e2580)
	/home/ericfreese/golang/src/github.com/ericfreese/rat/lib/pager.go:136 +0x11f
github.com/ericfreese/rat/lib.(*cmdPager).Render(0xc4200e2580)
	/home/ericfreese/golang/src/github.com/ericfreese/rat/lib/pager.go:144 +0x50
github.com/ericfreese/rat/lib.(*pagerStack).Render(0xc42005e200)
	/home/ericfreese/golang/src/github.com/ericfreese/rat/lib/pager_stack.go:170 +
github.com/ericfreese/rat/lib.(*widgetStack).Render(0xc42000c1c0)
	/home/ericfreese/golang/src/github.com/ericfreese/rat/lib/widget.go:68 +0x45
github.com/ericfreese/rat/lib.render()
	/home/ericfreese/golang/src/github.com/ericfreese/rat/lib/rat.go:168 +0x41
github.com/ericfreese/rat/lib.Run()
	/home/ericfreese/golang/src/github.com/ericfreese/rat/lib/rat.go:93 +0x6c
main.main()
	/home/ericfreese/golang/src/github.com/ericfreese/rat/main.go:65 +0x1df