stream: read at least a full buffer with stream_peek()

Until now, stream_peek() read only the bare minimum it had to read from
the stream. But this could cause problems, such as being very
inefficient when peeking a lot, or conflicting with ability to seek
back. (The latter issue can be caused by peeking a few bytes, and then
doing a stream_read() with a size that is 1 byte longer: this would read
the peeked data, then call stream_fill_buffer(), which throws away the
previously peeked bytes - so you can't seek back anymore. This is
mitigated by a hack in demux_open(): it peeks a full buffer, to avoid
that peeking/reading during demuxer probing [or before that, in a stream
filter] can cause the buffer to be dropped.)

stream/stream.c

@@ -550,7 +550,7 @@ struct bstr stream_peek(stream_t *s, int len)
         memmove(s->buffer, &s->buffer[s->buf_pos], buf_valid);
         // Fill rest of the buffer.
         while (buf_valid < len) {
-            int chunk = len - buf_valid;
+            int chunk = MPMAX(len - buf_valid, STREAM_BUFFER_SIZE);
             if (s->sector_size)
                 chunk = STREAM_BUFFER_SIZE;
             assert(buf_valid + chunk <= TOTAL_BUFFER_SIZE);