Fix the size formula for an allocation

[musicinmybrain]

Fixes #304 by adding one byte to the allocated size, based on the rationale offered by Jerry James, @jamesjer, in https://github.com/linbox-team/linbox/issues/304.

In addition, a new local variable is introduced for the size in question to reduce repetition.

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I tested this with -D_FORTIFY_SOURCE=3 and confirmed that the tests pass.