input/thread: set InputStream::ready after Open() failure

Without setting the "ready" flag, the caller will wait in WaitReady()
forever, locking up MPD.  Closes #252

src/input/ThreadInputStream.cxx

@@ -68,7 +68,7 @@ ThreadInputStream::ThreadFunc()
 		Open();
 	} catch (...) {
 		postponed_exception = std::current_exception();
-		cond.broadcast();
+		SetReady();
 		return;
 	}