Invalid superclass of mixin causes NullPointerException instead of InvalidMixinException

[Klotzi111]

This line https://github.com/SpongePowered/Mixin/blob/155314e6e91465dad727e621a569906a410cd6f4/src/main/java/org/spongepowered/asm/mixin/transformer/MixinInfo.java#L581 should be changed to this to fix this issue:

if (superClass != null && superClass.isMixin()) {