mbedtls_mpi_core_add_if() function

Conditional addition of two fixed-size large unsigned integers, returning the carry. Functionally equivalent to ``` if( cond ) X += A; return carry; ``` This function operates modulo `2^(biL*limbs)`. \warning If \p cond is neither 0 nor 1, the result of this function is unspecified, and the resulting value in \p X might be neither its original value nor \p X + \p A.