`ScriptBuf::extend` contained an optimization for short scripts that was
supposed to preallocate the buffer and then fill it. By mistake it
attempted to fill it from already-exhausted iterator instead of the
temporary array it drained the items into. This obviously produced
garbage (empty) values.
This was not caught by tests because the optimization is only active for
scripts with known maximum length and the test used `Instructions` which
doesn't know the maximum lenght.
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