Sidechain Limiter with 4 inputs segfaults if only 2 are connected

[JohannesLorenz]

The sidechain limiter has code that looks at all 4 input ports:

    inL = ins[0][offset];
    inR = ins[1][offset];
    scL = ins[2][offset];
    scR = ins[3][offset];

As 2 of them are optional, those should not be looked up if the respective ports are NULL. I guess the best fix would be to do it like in the sidechain compressor:

    scL = ins[2] ? ins[2][offset] : 0;
    scR = ins[3] ? ins[3][offset] : 0;

Thanks to @karmux for finding the segfault in an LMMS issue report.

[boomshop]

Thanks!