avformat/whip: check RTP history packet size before RTX retransmission

handle_rtx_packet() constructs an RTX packet by shifting the payload
of a history entry to insert the original sequence number. It uses
memmove with length (ori_size - 12), but never checks that ori_size
is at least 12 bytes (the minimum RTP header size).

Zero-initialized history slots have seq == 0 and size == 0.
rtp_history_find() only compares sequence numbers, so an RTCP NACK
requesting seq 0 early in a session matches such a slot. The
subtraction then wraps to a huge value when converted to size_t,
causing a stack buffer overflow in memmove().

Add a little size check to reject history entries smaller than and
valid RTP header before any arithmetic on their size.

Found-by: Pwno

libavformat/whip.c

@@ -1916,6 +1916,12 @@ static void handle_rtx_packet(AVFormatContext *s, uint16_t seq)
     ori_buf = it->buf;
     ori_size = it->size;
 
+    /* A valid RTP packet must have at least a RTP header. */
+    if (ori_size < WHIP_RTP_HEADER_SIZE) {
+        av_log(whip, AV_LOG_WARNING, "RTX history packet too small, size=%d\n", ori_size);
+        goto end;
+    }
+
     /* RTX packet format: header + original seq (2 bytes) + payload */
     if (ori_size + 2 > sizeof(rtx_buf)) {
         av_log(whip, AV_LOG_WARNING, "RTX packet is too large, size=%d\n", ori_size);