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>
