Added a return statement to avoid a compilation error.

It should be unreachable, but now the code compiles.

I'm new to Go so forgive me if this is the wrong solution, but it did make the compilation error go away. 

secp256.go:243[/tmp/go-build468925377/github.com/obscuren/secp256k1-go/_obj/secp256.cgo1.go:208]: function ends without a return statement