Fix panic 'integer divide by zero' when terminal width too low

[fxaguessy]

In awless, when the terminal width is too low we get a panic in readline:

panic: runtime error: integer divide by zero

goroutine 24 [running]:
github.com/wallix/awless/vendor/github.com/chzyer/readline.(*opCompleter).CompleteRefresh(0xc4200a4460)
	/Users/fx/go_workspace/src/github.com/wallix/awless/vendor/github.com/chzyer/readline/complete.go:206 +0x88e
github.com/wallix/awless/vendor/github.com/chzyer/readline.(*opCompleter).EnterCompleteMode(0xc4200a4460, 0x0, 0xc4200ea300, 0x18, 0x20)
	/Users/fx/go_workspace/src/github.com/wallix/awless/vendor/github.com/chzyer/readline/complete.go:269 +0x61
github.com/wallix/awless/vendor/github.com/chzyer/readline.(*opCompleter).OnComplete(0xc4200a4460, 0xc420b58a20)
	/Users/fx/go_workspace/src/github.com/wallix/awless/vendor/github.com/chzyer/readline/complete.go:109 +0x25b
github.com/wallix/awless/vendor/github.com/chzyer/readline.(*Operation).ioloop(0xc4200a4150)
	/Users/fx/go_workspace/src/github.com/wallix/awless/vendor/github.com/chzyer/readline/operation.go:178 +0xf16
created by github.com/wallix/awless/vendor/github.com/chzyer/readline.NewOperation
	/Users/fx/go_workspace/src/github.com/wallix/awless/vendor/github.com/chzyer/readline/operation.go:88 +0x34d

This is due to the fact that terminal width gets lower than colWidth, so colNum == 0. This PR fixes this bug.