Test Check multiplying by infinity returns infinity

[LLFourn]

see: https://github.com/bitcoin-core/secp256k1/pull/791/files

[elichai]

Note, that generally the high level API of libsecp256k1 Doesn't support points at infinity, because those aren't valid public keys and don't have any meaningful serialization

[LLFourn]

This is about the Low level API (ecmult and ecmult_const). It turns out that I was allowing infinity to be passed down to these and then panicking.

The solution is just to check for that and early exit. This is what gmax was suggesting here bitcoin-core/secp256k1#791 (I followed up suggesting it should be applied to ecmult as well).