segment: don't access outside seg->frames array

Fixes wrong number of segments output and undefined memory access.
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
(cherry picked from commit 58e0402e)
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavformat/segment.c

@@ -684,7 +684,7 @@ static int seg_write_packet(AVFormatContext *s, AVPacket *pkt)
         end_pts = seg->segment_count < seg->nb_times ?
             seg->times[seg->segment_count] : INT64_MAX;
     } else if (seg->frames) {
-        start_frame = seg->segment_count <= seg->nb_frames ?
+        start_frame = seg->segment_count < seg->nb_frames ?
             seg->frames[seg->segment_count] : INT_MAX;
     } else {
         if (seg->use_clocktime) {