dsa,ecdsa: Allocate secure memory for RFC6979 generation.

* cipher/dsa-common.c (_gcry_dsa_gen_rfc6979_k): Use secure memory
just like _gcry_dsa_gen_k does.

Cherry-picked master commit of:
75c2fbc43d2f2cf5f4c60cb28001fda7324185c2

• Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>