avformat/utils: Fix undefined behavior in ff_configure_buffers_for_index()

When e2_pts == INT64_MIN and e1_pts >= 0 the calculation of
e2_pts - e1_pts will overflow an int64_t.
Signed-off-by: Dale Curtis <dalecurtis@chromium.org>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavformat/utils.c

@@ -2108,6 +2108,8 @@ void ff_configure_buffers_for_index(AVFormatContext *s, int64_t time_tolerance)
     //We could use URLProtocol flags here but as many user applications do not use URLProtocols this would be unreliable
     const char *proto = avio_find_protocol_name(s->url);
 
+    av_assert0(time_tolerance >= 0);
+
     if (!proto) {
         av_log(s, AV_LOG_INFO,
                "Protocol name not provided, cannot determine if input is local or "
@@ -2135,7 +2137,7 @@ void ff_configure_buffers_for_index(AVFormatContext *s, int64_t time_tolerance)
                 for (; i2 < st2->nb_index_entries; i2++) {
                     AVIndexEntry *e2 = &st2->index_entries[i2];
                     int64_t e2_pts = av_rescale_q(e2->timestamp, st2->time_base, AV_TIME_BASE_Q);
-                    if (e2_pts - e1_pts < time_tolerance)
+                    if (e2_pts < e1_pts || e2_pts - (uint64_t)e1_pts < time_tolerance)
                         continue;
                     pos_delta = FFMAX(pos_delta, e1->pos - e2->pos);
                     break;