cipher: Fix possible NULL deref in call to prime generator.
* cipher/primegen.c (_gcry_generate_elg_prime): Change to return an error code. * cipher/dsa.c (generate): Take care of new return code. * cipher/elgamal.c (generate): Change to return an error code. Take care of _gcry_generate_elg_prime return code. (generate_using_x): Take care of _gcry_generate_elg_prime return code. (elg_generate): Propagate return code from generate.
Note that the NULL deref may have only happened on malloc failure.