bug#26021: 25.1; GDB/GUD tooltip error

[Eli Zaretskii]

> From: Live System User <nyc4bos@aol.com>
> Date: Tue, 07 Mar 2017 18:53:34 -0500
> 
>         Not sure how to get a reproducible recipe but while in
>         GDB when I do something like C-x C-f (find-file),
>         sometimes I get the folloeing error:
> 
> Debugger entered--Lisp error: (wrong-type-argument markerp nil)
>   marker-buffer(nil)
>   (eq (tooltip-event-buffer gud-tooltip-event) (marker-buffer 
> gud-overlay-arrow-position))
>   (and (eq (tooltip-event-buffer gud-tooltip-event) (marker-buffer 
> gud-overlay-arrow-position)))
>   eval((and (eq (tooltip-event-buffer gud-tooltip-event) (marker-buffer 
> gud-overlay-arrow-position))))
>   gud-tooltip-tips((mouse-movement (#<window 12 on *Backtrace*> 2258 (709 . 
> 70) 227820838 nil 2258 (74 . 3) nil (304 . 16) (9 . 18))))
>   run-hook-with-args-until-success(gud-tooltip-tips (mouse-movement (#<window 
> 12 on *Backtrace*> 2258 (709 . 70) 227820838 nil 2258 (74 . 3) nil (304 . 16) 
> (9 . 18))))
>   tooltip-timeout(nil)
>   apply(tooltip-timeout nil)
>   timer-event-handler([t 22719 16734 511876 nil tooltip-timeout (nil) nil 
> 929999])

Does it help to add the condition

   (markerp gud-overlay-arrow-position)

to the expression that is the value of the gud-tooltip-display
defcustom?