Update protostack.cpp

After calling push_front() on a std::deque<>, iterators become invalid per the C++ standard ("after push_back() and push_front() all references stays valid, but iterators don't"). 

Using prev_begin after the push_front() call results in undefined behavior. This fix removes the usage of an invalid iterator and makes the intent of the code more clear.
Update protostack.cpp

(fixed whitespace)
Merge branch '3.x' of https://github.com/AndreasVerhoeven/galera into AndreasVerhoeven-3.x
Merge branch 'AndreasVerhoeven-3.x' into 3.x