terminal/command: stack command panics when stack has 1 frame

Insure that the digits function always returns at least 1.

Fixes #354 (partial)

terminal/command.go

@@ -819,6 +819,9 @@ func (c *Commands) sourceCommand(t *Term, args string) error {
 }
 
 func digits(n int) int {
+	if n <= 0 {
+		return 1
+	}
 	return int(math.Floor(math.Log10(float64(n)))) + 1
 }