Fix a NULL dereference in chacha20_poly1305_init_key()

chacha20_poly1305_init_key() dereferences NULL when called with inkey !=
NULL && iv == NULL. This function is called by EVP_EncryptInit_ex()
family, whose documentation allows setting key and iv in separate calls.

Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Richard Levitte <levitte@openssl.org>