libipsec: Fix crypter lookup for AES-CTR

libipsec: Fix crypter lookup for AES-CTR

Due to the nonce, the ESP key material is four bytes longer than needed for
the actual AES key.  The crypto plugins, however, register their AES-CTR
implementations with the AES key length, so the lookup here failed.

For IKEv2 the key material is allocated after creating a crypter instance
with the negotiated AES key size.  The length of the actual key material is
retrieved via get_key_size(), which adds the four bytes to the AES key length.

Fixes #1124.