WebCrypto: return false for wrong-length ECDSA signatures

WebCrypto: return false for wrong-length ECDSA signatures

ECDSA signatures of an invalid length cause verify() to throw instead
of returning false.  njs passes an empty DER signature to
EVP_PKEY_verify(), while QuickJS rejects the length during conversion.

The njs behavior dates back to 86c202c7 (0.7.7); QuickJS rejects these
signatures since 75ca26f8 (0.8.10).

Treat a length mismatch as an invalid signature, as required by
WebCrypto ECDSA Verify, while preserving diagnostics for conversion
failures.  Test all three supported curves with invalid lengths and
a valid signature.