avdevice/lavfi: Don't require AV_PIX_FMT_NONE == -1

Reviewed-by: Nicolas George <george@nsup.org>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>

libavdevice/lavfi.c

@@ -76,7 +76,7 @@ static int *create_all_formats(int n)
         if (!(desc->flags & AV_PIX_FMT_FLAG_HWACCEL))
             fmts[j++] = i;
     }
-    fmts[j] = -1;
+    fmts[j] = AV_PIX_FMT_NONE;
     return fmts;
 }