input/last: clear "uri" in OnCloseTimer()

Without clearing the "uri" field, the next Open() call attempts to
reuse the old InputStream, but it has already been closed, so Open()
always returns nullptr.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/1300

NEWS

@@ -1,4 +1,6 @@
 ver 0.23.2 (not yet released)
+* protocol
+  - fix "albumart" timeout bug
 * input
   - nfs: fix playback bug
 * output

src/input/LastInputStream.cxx

@@ -42,5 +42,6 @@ LastInputStream::OnCloseTimer() noexcept
 {
 	assert(is);
 
+	uri.clear();
 	is.reset();
 }