The trick of subtracting one from the poitner returned from malloc
results in undefined behavior:

C89: 3.3.6 Unless both the pointer operand and the result point to a
member of the same array object, or one past the last member of the
array object, the behavior is undefined.

Instead, allocate 1 extra element and stop adjusting the pointer. While
a little wasteful, the extra is in the noise on today's systems.

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