PLL parameters are never going to be written as this check is bound to fail: pll_calc (a function pointer) will always differ from the null pointer (0).
Either:
The check is not needed, and PLL parameters always need to be written (probable, serious fault)
The check is needed, but the wrong comparison has been used (possible, serious fault)
The check is not needed, and PLL parameters never need to be written (doubtful, redundant code)
https://github.com/NT7S/Si5351/blob/1af6288d49458180d5aae45aab711aff31d16462/si5351.c#L138
PLL parameters are never going to be written as this check is bound to fail: pll_calc (a function pointer) will always differ from the null pointer (0).
Either: