In pk_sign_verify, if mbedtls_pk_sign() failed, sig_len was passed to
mbedtls_pk_verify_restartable() without having been initialized. This
worked only because in the only test case that expects signature to
fail, the verify implementation doesn't look at sig_len before failing
for the expected reason.
The value of sig_len if sign() fails is undefined, so set sig_len to
something sensible.
f85e4e67bd