avfilter/src_movie: fix how we check for overflows with seek_point

Currently, if the movie source filter is used and a seek_point is
specified on a file that has a negative start time, ffmpeg will fail.

An easy way to reproduce this is as follows:
$ ffmpeg -vsync passthrough -filter_complex 'color=d=10,setpts=PTS-1/TB' test.mp4
$ ffmpeg -filter_complex 'movie=filename=test.mp4:seek_point=2' -f null -

The problem is caused by checking for int64_t overflow the wrong way.
In general, to check whether a + b overflows, it is not enough to do:
    a > INT64_MAX - b
because b might be negative; the correct way is:
    b > 0 && > a > INT64_MAX - b
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavfilter/src_movie.c

@@ -240,7 +240,7 @@ static av_cold int movie_common_init(AVFilterContext *ctx)
         timestamp = movie->seek_point;
         // add the stream start time, should it exist
         if (movie->format_ctx->start_time != AV_NOPTS_VALUE) {
-            if (timestamp > INT64_MAX - movie->format_ctx->start_time) {
+            if (timestamp > 0 && movie->format_ctx->start_time > INT64_MAX - timestamp) {
                 av_log(ctx, AV_LOG_ERROR,
                        "%s: seek value overflow with start_time:%"PRId64" seek_point:%"PRId64"\n",
                        movie->file_name, movie->format_ctx->start_time, movie->seek_point);