avformat/sapdec: Check ffurl_get_file_handle() for error

Fixes: CID1604506 Overflowed constant

Sponsored-by: Sovereign Tech Fund
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit 3e305a0e70)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavformat/sapdec.c

@@ -196,6 +196,9 @@ static int sap_fetch_packet(AVFormatContext *s, AVPacket *pkt)
     struct pollfd p = {fd, POLLIN, 0};
     uint8_t recvbuf[RTP_MAX_PACKET_LENGTH];
 
+    if (fd < 0)
+        return fd;
+
     if (sap->eof)
         return AVERROR_EOF;