commit d7feb56 'fix panic on closing a tab' is causing another panic on closing a tab.

there is more than one way to reproduce the issue. here is one approach:

1. launch an up to date dev build (or a build that includes d7feb56)
2. open an email
3. open another tab (e.g. ctrl+t for a terminal tab)
4. go back to the prior tab with ctrl+p
5. close it
6. crash

without d7feb56 this bug cannot be reproduced with these steps.

panic: runtime error: index out of range [7] with length 7 [recovered]
	panic: runtime error: index out of range [7] with length 7

goroutine 1 [running]:
git.sr.ht/~rjarry/aerc/logging.PanicHandler()
	git.sr.ht/~rjarry/aerc/logging/panic-logger.go:47 +0x6de
panic({0xa69c00, 0xc00043e1c8})
	runtime/panic.go:844 +0x258
git.sr.ht/~rjarry/aerc/lib/ui.(*TabContent).Draw(0xc000440930?, 0xc00027f0e0?)
	git.sr.ht/~rjarry/aerc/lib/ui/tab.go:403 +0x145
git.sr.ht/~rjarry/aerc/lib/ui.(*Grid).Draw(0xc000150630, 0xc00019e2a0)
	git.sr.ht/~rjarry/aerc/lib/ui/grid.go:148 +0x2ff
git.sr.ht/~rjarry/aerc/widgets.(*Aerc).Draw(0xc00044a000, 0xc00019e2a0)
	git.sr.ht/~rjarry/aerc/widgets/aerc.go:182 +0x2e
git.sr.ht/~rjarry/aerc/lib/ui.(*UI).Tick(0xc0001800f0)
	git.sr.ht/~rjarry/aerc/lib/ui/ui.go:116 +0x1f7
main.main()
	git.sr.ht/~rjarry/aerc/aerc.go:228 +0xb74