pspp_coeff_var_to_coeff: Guard against a null pointer in coefs[i]->v_info.

pspp_coeff_var_to_coeff: Guard against a null pointer in coefs[i]->v_info.
regression.q: Pass variable lists to pspp_linreg_cache_alloc.
pspp_linreg_cache_alloc: Take variable lists as arguments to allow easier access
			 later and computation of number of coefficients during allocation.
rearrange_covariance_matrix: Use the new stored list of variables in the linreg_cache
			     instead of calling pspp_linreg_get_vars().
cache_init: Move computation of n_coeffs to pspp_linreg_cache_alloc().