video_display: fix use ptr after free

video_display putf implementation may free the frame so that the pointer
is no longer valid upon the exit from the callback (or anytime later).
This is the case for eg. vdisp/decklink.

Fixes `uv -t testcard -c lavc -d dummy:discard` (wrong color in output
line):
```
[dummy] 126 frames in 5.03501 seconds = 25.0248 FPS
```

even though FPS is +/- the nominal 25. Depending on circumstances it
could have also easily crashed (access after free).

Author: MartinPulec

src/video_display.c

@@ -351,6 +351,7 @@ static bool display_frame_helper(struct display *d, struct video_frame *frame, l
                 MIN_FPS_PERC_WARN  = 98,
                 MIN_FPS_PERC_WARN2 = 90,
         };
+        const double frame_fps = frame->fps;
         bool ret = d->funcs->putf(d->state, frame, timeout_ns);
         if (!d->funcs->generic_fps_indicator_prefix) {
                 return ret;
@@ -364,7 +365,7 @@ static bool display_frame_helper(struct display *d, struct video_frame *frame, l
         if (seconds_ns > 5 * NS_IN_SEC) {
                 const double seconds = (double) seconds_ns / NS_IN_SEC;
                 const double fps      = d->frames / seconds;
-                const char *const fps_col  = get_stat_color(fps / frame->fps);
+                const char *const fps_col  = get_stat_color(fps / frame_fps);
 
                 log_msg(LOG_LEVEL_INFO,
                         TERM_BOLD TERM_FG_MAGENTA