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.
diff --git a/ChangeLog b/ChangeLog
index b0a01c0..8d10ea2 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -18,6 +18,9 @@
      Fixes #1040.
    * Fix C89 incompatibility in benchmark.c. Contributed by Brendan Shanks.
      #1353
+   * Fix parsing of PKCS#8 encoded Elliptic Curve keys. Previously Mbed TLS was
+     unable to parse keys with only the optional parameters field of the
+     ECPrivateKey structure. Found by jethrogb, fixed in #1379.
 
 Changes
    * Support cmake build where Mbed TLS is a subproject. Fix