Bugs in scratch allocator

[jkunstwald]

I've found two bugs in the scratch allocator (memory.cpp):

1. If the ring buffer head (_allocate) ends up exactly at the end of the buffer after a call to allocate, it will never be reached by the while-loop in deallocate - the last condition in that loop checks equality with _end and wraps around to _begin - resulting in an infinite loop.
   This is fixed by if (_allocate == _end) { _allocate = _begin; } before returning.

2. Allocating more memory than available incorrectly returns an OOB pointer if the ring buffer is otherwise empty, because the check is based on in_use(p), which immediately returns false if _free == _allocate.
   This can be fixed by extending the condition to if (p > _end || in_use(p)).

[jkunstwald]

As a follow up, there is at least one additional bug but i haven't tracked it down, the symptoms are misdiagnosed double-frees and memory corruption. I wouldn't recommend using this allocator