GEAR: Fix improper handling of IMF transition in star_population_continuous_IMF

We found a bug in the feedback for the GEAR star_population_continuous_IMF type stars. Specifically, the transition between the discrete and continuous part of the IMF (in the code, the IMF is split by sm->imf.minimal_discrete_mass_Msun) was not properly managed in the cases where m_beg_step > sm->imf.minimal_discrete_mass_Msun > m_end_step. This edge case is now properly accounted for, ensuring a smooth and accurate transition between the two IMF regimes.