aviobuf: Handle a NULL buffer in avio_close_dyn_buf

This simplifies proper error handling in rtsp.c/rtspdec.c. When
broadcasting over RTSP in TCP mode, the AVIOContext is closed and
recreated for each sent packet, and if the recreation fails, we might
try to close a NULL buffer when freeing things at the end.

Previously, if recreating the buffer in rtspdec.c failed, this would
crash later due to trying to close a NULL buffer.
Signed-off-by: Martin Storsjö <martin@martin.st>

libavformat/aviobuf.c

@@ -964,6 +964,11 @@ int avio_close_dyn_buf(AVIOContext *s, uint8_t **pbuffer)
     static const char padbuf[FF_INPUT_BUFFER_PADDING_SIZE] = {0};
     int padding = 0;
 
+    if (!s) {
+        *pbuffer = NULL;
+        return 0;
+    }
+
     /* don't attempt to pad fixed-size packet buffers */
     if (!s->max_packet_size) {
         avio_write(s, padbuf, sizeof(padbuf));