Fix possible infinite loop

[aido]

Hi,

I am not sure why the share_length and secret_length variables are defined as type uint32_t but both are being compared to a uint8_t in a loop condition. This causes CodeQL to give a high severity security warning. 'High Severity' is probably a bit alarmist but in a loop condition, comparison of a value of a narrow type with a value of a wide type may result in unexpected behaviour if the wider value is sufficiently large (or small). This is because the narrower value may overflow. This can lead to an infinite loop.

See here for further explanation of the warning:
Comparison of narrow type with wide type in loop condition

This PR fixes #42 by casting share_length and secret_length to uint8_t ensuring comparison of same types. If type casting is not the solution then maybe share_length and secret_length should be declared as uint8_t not uint32_t?

Note: I chose to cast to the narrower uint8_t rather than declaring j as the wider uint32_t as I think neither share_length or secret_length should ever be bigger than 255.