alsa: do not leak memory on failure to read frames from pcm device.

(cherry picked from commit c9ed94876c9b6ea3b09c2131db13388ae26e49f1)
Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>

modules/access/alsa.c

@@ -185,6 +185,7 @@ static void *Thread (void *data)
         pts = mdate ();
         if (frames < 0)
         {
+            block_Release (block);
             if (frames == -EAGAIN)
                 continue;