Some more EC pubkey parsing refactoring

Fix a bug in pk_rsa() and pk_ec() along the way
diff --git a/tests/suites/test_suite_x509parse.data b/tests/suites/test_suite_x509parse.data
index bbfbed4..77f31d4 100644
--- a/tests/suites/test_suite_x509parse.data
+++ b/tests/suites/test_suite_x509parse.data
@@ -523,7 +523,7 @@
 x509parse_crt:"30693067a0030201028204deadbeef300d06092a864886f70d0101020500300c310a30080600130454657374301c170c303930313031303030303030170c303931323331323335393539300c310a300806001304546573743011300d06092A864886F70D01010105000300":"":POLARSSL_ERR_X509_CERT_INVALID_PUBKEY + POLARSSL_ERR_ASN1_OUT_OF_DATA
 
 X509 Certificate ASN1 (TBSCertificate, pubkey, invalid bitstring start)
-x509parse_crt:"306a3068a0030201028204deadbeef300d06092a864886f70d0101020500300c310a30080600130454657374301c170c303930313031303030303030170c303931323331323335393539300c310a300806001304546573743012300d06092A864886F70D0101010500030101":"":POLARSSL_ERR_X509_CERT_INVALID_PUBKEY
+x509parse_crt:"306a3068a0030201028204deadbeef300d06092a864886f70d0101020500300c310a30080600130454657374301c170c303930313031303030303030170c303931323331323335393539300c310a300806001304546573743012300d06092A864886F70D0101010500030101":"":POLARSSL_ERR_X509_CERT_INVALID_PUBKEY + POLARSSL_ERR_ASN1_INVALID_DATA
 
 X509 Certificate ASN1 (TBSCertificate, pubkey, invalid internal bitstring length)
 x509parse_crt:"306d306ba0030201028204deadbeef300d06092a864886f70d0101020500300c310a30080600130454657374301c170c303930313031303030303030170c303931323331323335393539300c310a300806001304546573743015300d06092A864886F70D0101010500030400300000":"":POLARSSL_ERR_X509_CERT_INVALID_PUBKEY + POLARSSL_ERR_ASN1_LENGTH_MISMATCH
@@ -595,7 +595,7 @@
 x509parse_crt:"308192308180a0030201008204deadbeef300d06092a864886f70d0101020500300c310a30080600130454657374301c170c303930313031303030303030170c303931323331323335393539300c310a30080600130454657374302a300d06092A864886F70D010101050003190030160210ffffffffffffffffffffffffffffffff0202ffff300d06092a864886f70d0101020500":"":POLARSSL_ERR_X509_CERT_INVALID_SIGNATURE + POLARSSL_ERR_ASN1_OUT_OF_DATA
 
 X509 Certificate ASN1 (signature, invalid sig data)
-x509parse_crt:"308195308180a0030201008204deadbeef300d06092a864886f70d0101020500300c310a30080600130454657374301c170c303930313031303030303030170c303931323331323335393539300c310a30080600130454657374302a300d06092A864886F70D010101050003190030160210ffffffffffffffffffffffffffffffff0202ffff300d06092a864886f70d0101020500030100":"":POLARSSL_ERR_X509_CERT_INVALID_SIGNATURE
+x509parse_crt:"308195308180a0030201008204deadbeef300d06092a864886f70d0101020500300c310a30080600130454657374301c170c303930313031303030303030170c303931323331323335393539300c310a30080600130454657374302a300d06092A864886F70D010101050003190030160210ffffffffffffffffffffffffffffffff0202ffff300d06092a864886f70d0101020500030100":"":POLARSSL_ERR_X509_CERT_INVALID_SIGNATURE + POLARSSL_ERR_ASN1_INVALID_DATA
 
 X509 Certificate ASN1 (signature, data left)
 x509parse_crt:"308197308180a0030201008204deadbeef300d06092a864886f70d0101020500300c310a30080600130454657374301c170c303930313031303030303030170c303931323331323335393539300c310a30080600130454657374302a300d06092A864886F70D010101050003190030160210ffffffffffffffffffffffffffffffff0202ffff300d06092a864886f70d0101020500030200ff00":"":POLARSSL_ERR_X509_CERT_INVALID_FORMAT + POLARSSL_ERR_ASN1_LENGTH_MISMATCH