Crash in Exceptions::log_exception() due erroneous copying of stringStream

(Yet unpushed) JDK-8224193 uncovered a bug in Exceptions::log_exception(). Stream object argument is handed down by value:

void Exceptions::log_exception(Handle exception, stringStream tempst);

which creates a copy which shares the internal backing buffer with the original one frame upstairs. Worked out of accident before when backing buffer was allocated in resource area, but since with JDK-8224193, it is allocated via C-heap which leads to a double free.