win-wasapi: Initialize a pointer reroute_target

The pointer `reroute_target` was never initialized if the reroute was
not set but read during processing audio.

plugins/win-wasapi/win-wasapi.cpp

@@ -161,7 +161,7 @@ class WASAPISource {
 	ComPtr<IAudioCaptureClient> capture;
 
 	obs_source_t *source;
-	obs_source_t *reroute_target;
+	obs_source_t *reroute_target = nullptr;
 	wstring default_id;
 	string device_id;
 	string device_name;