BUG/MINOR: jwt: enforce the ECDSA signature length in jwt_verify

BUG/MINOR: jwt: enforce the ECDSA signature length in jwt_verify

RFC 7518 section 3.4 states that an ES256 signature MUST be a 64-octet
sequence made of R and S each on 32 bytes, and that validation fails
otherwise (96 bytes for ES384 and 132 for ES512). jwt_verify() and
jwt_verify_cert() accept any even length and split it in two halves, so
that R and S may be prefixed with any number of zero bytes.
This can be reproduced by taking a valid ES256 token, decoding its
signature, inserting a zero byte before each 32-byte half, and encoding it
again: the converter still returns 1.

Let's check that the signature has exactly the length mandated by the
algorithm in convert_ecdsa_sig(), and return JWT_VRFY_INVALID_TOKEN (-3)
otherwise, as was already done for odd lengths. A reg-test is provided.

This fix should be backported to all supported versions.