Correctly reduce negative inpot to BN_mod_exp2_mont()

Correctly reduce negative inpot to BN_mod_exp2_mont()

Negative bases could result in a negative modulus being returned. This is
not strictly speaking incorrect but slightly surprising. This is all a
consequence of the shortcut of defining BN_mod() as a macro using BN_div().

Fixes ossfuzz #55997

ok jsing