Fix buggy memcpy in DstreamRead

Signed-off-by: Rémi Denis-Courmont <remi@remlab.net>
(cherry picked from commit 49482677)

src/input/stream_demux.c

@@ -161,6 +161,7 @@ static int DStreamRead( stream_t *s, void *p_read, unsigned int i_read )
             i_copy = __MIN( i_read, p_block->i_buffer );
             if( p_out && i_copy ) memcpy( p_out, p_block->p_buffer, i_copy );
             i_read -= i_copy;
+            p_out += i_copy;
             i_out += i_copy;
             p_block->i_buffer -= i_copy;
             p_block->p_buffer += i_copy;