[ramlakhanmadheshiya]

New components from addat! start at val (default zero) in u and the step history.
idxs must be the next indices, since the state buffers only append.
The default solver now resizes its stiff sub-caches too.

using StochasticDiffEq
f!(du, u, p, t) = (du .= -0.5 .* u; nothing)
g!(du, u, p, t) = (du .= 0.1 .* u; nothing)
integ = init(SDEProblem(f!, g!, [1.0, 2.0], (0.0, 1.0)), EM(); dt = 1 / 64, seed = 1)
addat!(integ, 3:3, [1.0])
solve!(integ)
integ.u[3]  # 0.7600697513197398

Fixes #4828