grid: don't draw at a negative offset

aerc panics when using macOS's default terminal emulator, Terminal.app,
when closing all but aerc's tab:

This error was also written to: /tmp/aerc-crash-20220427-194134.log
panic: Attempted to create context with negative offset [recovered]
	panic: Attempted to create context with negative offset

goroutine 1 [running]:
git.sr.ht/~rjarry/aerc/logging.PanicHandler()
	/Users/ckuehl/src/aerc/logging/panic-logger.go:47 +0x58c
panic({0x100d077a0, 0x14000032700})
	/opt/homebrew/Cellar/go/1.18.1/libexec/src/runtime/panic.go:844 +0x258
git.sr.ht/~rjarry/aerc/lib/ui.(*Context).Subcontext(0x1400013e420?, 0x14000202360?, 0x140000ffc48?, 0x1009a10e4?, 0x100da9440?)
	/Users/ckuehl/src/aerc/lib/ui/context.go:47 +0x160
git.sr.ht/~rjarry/aerc/lib/ui.(*Grid).Draw(0x1400013e420, 0x14000202360)
	/Users/ckuehl/src/aerc/lib/ui/grid.go:143 +0x2bc
git.sr.ht/~rjarry/aerc/widgets.(*Aerc).Draw(0x1400013e4d0, 0x14000202360)
	/Users/ckuehl/src/aerc/widgets/aerc.go:178 +0x30
git.sr.ht/~rjarry/aerc/lib/ui.(*UI).Tick(0x1400022bcc0)
	/Users/ckuehl/src/aerc/lib/ui/ui.go:116 +0x248
main.main()
	/Users/ckuehl/src/aerc/aerc.go:226 +0x9e8

I'm not entirely sure what the interactions are between the terminal
emulator, aerc's grid, and the space that moves around when the tab bar
disappears because there are no more tabs, but this fixes the issue 100%
of the time and I haven't noticed any issues.

Signed-off-by: Connor Kuehl <cipkuehl@gmail.com>
Acked-by: Robin Jarry <robin@jarry.cc>

lib/ui/grid.go

@@ -123,6 +123,10 @@ func (grid *Grid) Draw(ctx *Context) {
 		cols := grid.columnLayout[cell.Column : cell.Column+cell.ColSpan]
 		x := cols[0].Offset
 		y := rows[0].Offset
+		if x < 0 || y < 0 {
+			continue
+		}
+
 		width := 0
 		height := 0
 		for _, col := range cols {