Fix panic when use header border without pointer/marker

[phanen]

FZF_DEFAULT_OPTS= FZF_DEFAULT_OPTS_FILE= fzf --header-border --pointer= --header-lines 1

panic: strings: negative Repeat count

goroutine 58 [running]:
strings.Repeat({0x5c5f0633ad10?, 0x5c5f0633ad10?}, 0x1?)
	strings/strings.go:624 +0x585
github.com/junegunn/fzf/src.(*Terminal).printHeaderImpl(0xc000190008, {0x5c5f063a3978, 0xc00032c280?}, 0x4, {0x5c5f0656f960, 0x0, 0x5c5f062cde68?}, {0xc000114010, 0x1, 0x1})
	github.com/junegunn/fzf/src/terminal.go:2655 +0x1c6
github.com/junegunn/fzf/src.(*Terminal).printHeader.func1(...)
	github.com/junegunn/fzf/src/terminal.go:2610
github.com/junegunn/fzf/src.(*Terminal).withWindow(...)
	github.com/junegunn/fzf/src/terminal.go:1269
github.com/junegunn/fzf/src.(*Terminal).printHeader(0xc000190008)
	github.com/junegunn/fzf/src/terminal.go:2605 +0xe6
github.com/junegunn/fzf/src.(*Terminal).Loop.func8.2(0xc000010810)
	github.com/junegunn/fzf/src/terminal.go:4656 +0x565
github.com/junegunn/fzf/src/util.(*EventBox).Wait(0xc000010810, 0xc0002c7fa0)
	github.com/junegunn/fzf/src/util/eventbox.go:34 +0x5d
github.com/junegunn/fzf/src.(*Terminal).Loop.func8()
	github.com/junegunn/fzf/src/terminal.go:4581 +0xd7
created by github.com/junegunn/fzf/src.(*Terminal).Loop in goroutine 51
	github.com/junegunn/fzf/src/terminal.go:4559 +0x5fc

[junegunn]

Nice catch, thanks!