Thanks @mmkeeper

This could only happen if the maximum SIZE_T is not at least 2 times
bigger than INT_MAX. Not sure if this can happen on real systems, but
better be safe then sorry.

This fixes -Wimplicit-fallthrough warnings with GCC7.

This is used to update the buffer offset and determine success

This is necessary, because you can get it wrong if you do it manually.
(when you forget the & 0xFF in the comparison)

This is the first step in removing the global allocator functions. Every
internal function now only accesses its locally available set of
allocators.