switch-account: fix out-of-bounds panic

Fix out-of-bounds panic by updating the focused int variable when
headers change in the switch-account commands.

Fixes: d371c1ac8 ("commands: add switch-account command for composer")
Reported-by: Bence Ferdinandy <bence@ferdinandy.com>
Signed-off-by: Koni Marti <koni.marti@gmail.com>
Acked-by: Robin Jarry <robin@jarry.cc>

widgets/compose.go

@@ -176,6 +176,9 @@ func (c *Composer) setupFor(acct *AccountView) error {
 	if focusEditor != nil {
 		c.focusable = append(c.focusable, focusEditor)
 	}
+	if c.focused >= len(c.focusable) {
+		c.focused = len(c.focusable) - 1
+	}
 
 	// redraw the grid
 	c.updateGrid()