Modifies the first argument in place to hold the result of
multiplying by a numeric scalar. CryptoContextImpl only
declares EvalMultInPlace scalar overloads in the v1.5.1.0
header surface — the ct/ct and ct/pt variants that
upstream-defects P1 refers to live on SchemeBase and are not
exposed on CryptoContextImpl, so R's eval_mult_in_place
supports only the scalar case. The ct/ct multiplication
continues to work via the non-in-place eval_mult() generic.