nutdec: check for negative frame rate in decode_info_header

A negative frame rate triggers an av_assert2 in av_rescale_rnd.
Signed-off-by: Andreas Cadhalpun <Andreas.Cadhalpun@googlemail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavformat/nutdec.c

@@ -553,7 +553,8 @@ static int decode_info_header(NUTContext *nut)
 
             if (stream_id_plus1 && !strcmp(name, "r_frame_rate")) {
                 sscanf(str_value, "%d/%d", &st->r_frame_rate.num, &st->r_frame_rate.den);
-                if (st->r_frame_rate.num >= 1000LL*st->r_frame_rate.den)
+                if (st->r_frame_rate.num >= 1000LL*st->r_frame_rate.den ||
+                    st->r_frame_rate.num < 0 || st->r_frame_rate.num < 0)
                     st->r_frame_rate.num = st->r_frame_rate.den = 0;
                 continue;
             }