Ensure we aren't selecting negative directories

When the list of directories is empty trying to navigate in the
directory list did previously lead to a crash. With this change we
instead return early before trying to change the directory.

Example backtrace:

> panic: runtime error: index out of range [-1]
>
> goroutine 1 [running]:
> git.sr.ht/~sircmpwn/aerc/widgets.(*DirectoryList).NextPrev(0xc000160680, 0xffffffffffffffff)
> 	source/aerc/widgets/dirlist.go:285 +0xd4
> git.sr.ht/~sircmpwn/aerc/commands/account.NextPrevFolder.Execute(0xc000191040, 0xc00025c210, 0x1, 0x1, 0x0, 0xc00016f420)
> 	source/aerc/commands/account/next-folder.go:44 +0xe0
> git.sr.ht/~sircmpwn/aerc/commands.(*Commands).ExecuteCommand(0xc0000101a8, 0xc000191040, 0xc00025c210, 0x1, 0x1, 0xc000020070, 0xb46d01)
> 	source/aerc/commands/commands.go:66 +0xa7
> main.execCommand(0xc000191040, 0xc0001ca190, 0xc00025c210, 0x1, 0x1, 0xc00025c210, 0xc0003fb080)
> 	source/aerc/aerc.go:60 +0xc7
> main.main.func3(0xc00025c210, 0x1, 0x1, 0x1, 0x1)
> 	source/aerc/aerc.go:162 +0x57
> git.sr.ht/~sircmpwn/aerc/widgets.(*Aerc).BeginExCommand.func1(0xc000201db0, 0xb)
> 	source/aerc/widgets/aerc.go:382 +0x83
> git.sr.ht/~sircmpwn/aerc/widgets.(*ExLine).Event(0xc0003be100, 0xb475a0, 0xc00023cba0, 0xc00023cba0)
> 	source/aerc/widgets/exline.go:79 +0x131
> git.sr.ht/~sircmpwn/aerc/widgets.(*Aerc).Event(0xc000191040, 0xb475a0, 0xc00023cba0, 0x99ee01)
> 	source/aerc/widgets/aerc.go:202 +0x4c1
> git.sr.ht/~sircmpwn/aerc/widgets.(*Aerc).simulate(0xc000191040, 0xc000036f00, 0xd, 0x10)
> 	source/aerc/widgets/aerc.go:195 +0x8d
> git.sr.ht/~sircmpwn/aerc/widgets.(*Aerc).Event(0xc000191040, 0xb475a0, 0xc00023c9c0, 0x9c5a60)
> 	source/aerc/widgets/aerc.go:218 +0x3e8
> git.sr.ht/~sircmpwn/aerc/lib/ui.(*UI).Tick(0xc0001ca190, 0xa99d00)
> 	source/aerc/lib/ui/ui.go:92 +0x190
> main.main()
> 	source/aerc/aerc.go:192 +0x5f2

0 files changed, 0 insertions, 0 deletions