Fix parsing of PKCS#8 encoded Elliptic Curve keys.

The relevant ASN.1 definitions for a PKCS#8 encoded Elliptic Curve key are:

PrivateKeyInfo ::= SEQUENCE {
  version                   Version,
  privateKeyAlgorithm       PrivateKeyAlgorithmIdentifier,
  privateKey                PrivateKey,
  attributes           [0]  IMPLICIT Attributes OPTIONAL
}

AlgorithmIdentifier  ::=  SEQUENCE  {
  algorithm   OBJECT IDENTIFIER,
  parameters  ANY DEFINED BY algorithm OPTIONAL
}

ECParameters ::= CHOICE {
  namedCurve         OBJECT IDENTIFIER
  -- implicitCurve   NULL
  -- specifiedCurve  SpecifiedECDomain
}

ECPrivateKey ::= SEQUENCE {
  version        INTEGER { ecPrivkeyVer1(1) } (ecPrivkeyVer1),
  privateKey     OCTET STRING,
  parameters [0] ECParameters {{ NamedCurve }} OPTIONAL,
  publicKey  [1] BIT STRING OPTIONAL
}

Because of the two optional fields, there are 4 possible variants that need to
be parsed: no optional fields, only parameters, only public key, and both
optional fields. Previously mbedTLS was unable to parse keys with "only
parameters". Also, only "only public key" was tested. There was a test for "no
optional fields", but it was labelled incorrectly as SEC.1 and not run because
of a great renaming mixup.

library/pkparse.c

@@ -861,7 +861,10 @@ static int pk_parse_key_sec1_der( mbedtls_ecp_keypair *eck,
             mbedtls_ecp_keypair_free( eck );
             return( MBEDTLS_ERR_PK_KEY_INVALID_FORMAT + ret );
         }
+    }
 
+    if( p != end )
+    {
         /*
          * Is 'publickey' present? If not, or if we can't read it (eg because it
          * is compressed), create it from the private key.