fix: handle odd keys

[dignifiedquire]

Ther are two issues with oddly formed keys that were not properly handled

• avoid using - to avoid a subtraction with overflow for pkcs
• always validate the key in from_components to avoid errors in the internal precompute

[tarcieri]

I’m reminded of #445

[dignifiedquire]

yes, but the goal here is not general security, but rather for a first step remove unusable keys

once we have a solution for #445 this can be folded in

[dignifiedquire]

already published on the 0.9.x branch, so merging