fix: prevent silent overflow in calloc

[stringhandler]

Minor overflow prevention.

Overflow can silently occur in release builds. Low priority but simple enough fix.

[delta1]

ACK 11153bb; tested locally

[apoelstra]

In 11153bb:

In theory this is still bad because it will allocate less memory than the user expects, which could lead to writing past the end of the buffer. (Or could it? I suppose that the user can't actually write more memory than is addressable, which this saturation should limit at..)

[apoelstra]

ACK 11153bb; successfully ran local tests

[stringhandler]

It gets saturating_added later anyway

rust-simplicity/simplicity-sys/src/alloc.rs

Line 52 in 160d782

let size_prefixed_bytes = MIN_ALIGN.saturating_add(size_bytes);