pipeline::fetch: Do not pass a temporary to boost::any_cast

boost::any_cast<T>(const any<T> &) coerces the payload to be const,
which is invalid if the wrapped object was not originally const.

This happens e.g. when boost::any contains an int which is wrapped in a
boost::any::holder<int>;
boost::any_cast<int>(const any<int> &) will try to coerce this into a
boost::any::holder<const int>
which results in undefined behavior.

tpie/pipelining/pipeline.h

@@ -181,7 +181,8 @@ class pipeline {
 
 	template <typename T>
 	T fetch(std::string key) {
-		return boost::any_cast<T>(fetch_any(key));
+		boost::any a = fetch_any(key);
+		return *boost::any_cast<T>(&a);
 	}
 
 	void forward_any(std::string key, const boost::any & value) {