Changing coefficient with NaN or Inf as old value doesn't work

[tkoolen]

From https://github.com/tkoolen/Parametron.jl/issues/104,

https://github.com/JuliaOpt/LinQuadOptInterface.jl/blob/7f4d09bf834f1ba3e2938397dbfae5c7108fe281/src/constraints/scalaraffine.jl#L264

means that if previous.constant is NaN or Inf, the value that is passed to change_rhs_coefficient! is incorrect.

See also https://github.com/oxfordcontrol/OSQP.jl/issues/54.

CC: @rdeits.