Avoid outputting invalid signatures

Avoid outputting invalid signatures

The caller can provide an r which will be added to the ECDSA_SIG unchecked.
This can happen via ECDSA_{,do_}sign_ex() or ECDSA_sign_setup() or else via
a custom sign_sig() handler. Therefore add a check that it is in the bounds
required.

Since k was long thrown away, there's no way to check kinv, so it needs to
be trusted. Misdesigned APIs that will output garbage everywhere...

ok jsing