Add ecdsa_from_keypair()

Also fix bug/limitation in mpi_copy: would segfault if src just initialised
and not set to a value yet. (This case occurs when copying a context which
contains only the public part of the key, eg.)

include/polarssl/ecdsa.h

@@ -142,6 +142,16 @@ int ecdsa_read_signature( ecdsa_context *ctx,
 int ecdsa_genkey( ecdsa_context *ctx, ecp_group_id gid,
                   int (*f_rng)(void *, unsigned char *, size_t), void *p_rng );
 
+/**
+ * \brief           Set an ECDSA context from an EC key pair
+ *
+ * \param ctx       ECDSA context to set
+ * \param key       EC key to use
+ *
+ * \return          0 on success, or a POLARSSL_ERR_ECP code.
+ */
+int ecdsa_from_keypair( ecdsa_context *ctx, const ecp_keypair *key );
+
 /**
  * \brief           Initialize context
  *