Fix wrong ECDSA algorithm-identifier

[smhmeier]

RFC 5758 specifies in section 3.2 ECDSA Signature Algorithm the following:

When the ecdsa-with-SHA224, ecdsa-with-SHA256, ecdsa-with-SHA384, or
ecdsa-with-SHA512 algorithm identifier appears in the algorithm field
as an AlgorithmIdentifier, the encoding MUST omit the parameters
field.

Using V_ASN1_NULL adds the NULL parameter 30 0C 06 08 2A 86 48 CE 3D 04 03 02 05 00
Decoded:

SEQUENCE (2 elem)
     OBJECT IDENTIFIER 1.2.840.10045.4.3.2 ecdsaWithSHA256 (ANSI X9.62 ECDSA algorithm with SHA256)
    [0] [?] NULL 

Using V_ASN1_UNDEF creates the desired ASN1 structure 30 0A 06 08 2A 86 48 CE 3D 04 03 02:

SEQUENCE (1 elem)
    OBJECT IDENTIFIER 1.2.840.10045.4.3.2 ecdsaWithSHA256 (ANSI X9.62 ECDSA algorithm with SHA256)