avformat/aviobuf: Do not resize the buffer in ffio_ensure_seekback() if there is no read function

Without a read function there can never be more data read than the buffer could
hold
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavformat/aviobuf.c

@@ -805,7 +805,7 @@ int ffio_ensure_seekback(AVIOContext *s, int64_t buf_size)
 
     buf_size += s->buf_ptr - s->buffer + max_buffer_size;
 
-    if (buf_size < filled || s->seekable)
+    if (buf_size < filled || s->seekable || !s->read_packet)
         return 0;
     av_assert0(!s->write_flag);