output/alsa: clear the `period_buffer` in LockCaughtError()

The method Cancel() assumes that the `period_buffer` must be empty
when `active==false`, but that is not the case when Play() fails.

Of course the assertion in Cancel() is not 100% correct, but I decided
to rather fix this in LockCaughtError() because the `period_buffer`
should only be accessed from within the RTIO thread, and this is the
only code path where `active` can be set to `false` with a non-empty
`period_buffer`.

Fixes #423

NEWS

@@ -1,4 +1,6 @@
 ver 0.21.3 (not yet released)
+* output
+  - alsa: fix crash bug
 
 ver 0.21.2 (2018/11/12)
 * protocol

src/output/plugins/AlsaOutputPlugin.cxx

@@ -306,6 +306,8 @@ private:
 	}
 
 	void LockCaughtError() noexcept {
+		period_buffer.Clear();
+
 		const std::lock_guard<Mutex> lock(mutex);
 		error = std::current_exception();
 		active = false;