Don't free the front buffer if screencopy fails

It is possible for `screencopy_failed` to be called without
the front buffer ever being set, such as when the output is dpms-off.

Fixes #65

src/screencopy.c

@@ -191,6 +191,7 @@ static void screencopy_failed(void* data,
 
 	screencopy__stop(self);
 
+	if (self->front)
 		wv_buffer_pool_release(self->pool, self->front);
 	self->front = NULL;