prevent potential memleaks in Arena::Allocate*()
prevent potential memleaks in Arena::Allocate*()

The previous memory allocation procedures tried to allocate memory
via `new` or `mmap` and inserted the pointer to the memory into an
std::vector afterwards. In case `new` or `mmap` threw or returned
a nullptr, no memory was leaking. If `new` or `mmap` worked ok, the
following `vector::push_back` could still fail and throw an exception.
In this case, the memory just allocated was leaked.

The fix is to reserve space in the target memory pointer block
beforehand. If this throws, then no memory is allocated nor leaked.
If the reserve works but the actual allocation fails, still no
memory is leaked, only the target vector will have space for at
least one more element than actually required (but this may be
reused for the next allocation)

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