Avoid future chance of out-of-bounds error #540
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Regarding issue #529: do not assume that more than enough memory is automatically allocated and set to
0.The old code assumes that there is always enough memory allocated to go one further than
x.dp[x.used - 1]andthat this memory is set to zero. This can lead to surprises if e.g.: heap is short but not short enough to use tomsfastmath and the developer decides to pepper the code with
mp_shrink()to give the OS a chance to reclaim memory. If that is done ins_mp_sub()the assumptions fail and either an out-of-bounds error is caught or memory read with random content which can be zero. Depending on the input. And the compiler.It is not much, but let's be nice to the future developers.
Cost involved: one
mp_digit, its assignment and one branch (a ternary).This fixes #529