flvdec: Add sanity checking of the last packet size

For http, this avoids spurious warnings about failed requests (e.g.
HTTP error 416 Requested Range Not Satisfiable), if the last packet
is truncated and the size read is bogus.
Signed-off-by: Martin Storsjö <martin@martin.st>

libavformat/flvdec.c

@@ -846,13 +846,15 @@ skip:
         const int64_t fsize = avio_size(s->pb);
         avio_seek(s->pb, fsize - 4, SEEK_SET);
         size = avio_rb32(s->pb);
-        // Seek to the start of the last FLV tag at position (fsize - 4 - size)
-        // but skip the byte indicating the type.
-        avio_seek(s->pb, fsize - 3 - size, SEEK_SET);
-        if (size == avio_rb24(s->pb) + 11) {
-            uint32_t ts = avio_rb24(s->pb);
-            ts         |= avio_r8(s->pb) << 24;
-            s->duration = ts * (int64_t)AV_TIME_BASE / 1000;
+        if (size > 0 && size < fsize) {
+            // Seek to the start of the last FLV tag at position (fsize - 4 - size)
+            // but skip the byte indicating the type.
+            avio_seek(s->pb, fsize - 3 - size, SEEK_SET);
+            if (size == avio_rb24(s->pb) + 11) {
+                uint32_t ts = avio_rb24(s->pb);
+                ts         |= avio_r8(s->pb) << 24;
+                s->duration = ts * (int64_t)AV_TIME_BASE / 1000;
+            }
         }
         avio_seek(s->pb, pos, SEEK_SET);
         flv->searched_for_end = 1;