pki_crypto: Use explicit_bzero()

Signed-off-by: Andreas Schneider <asn@cryptomilk.org>
Reviewed-by: Jakub Jelen <jjelen@redhat.com>
(cherry picked from commit 1acb82e38a)

src/pki_crypto.c

@@ -1230,7 +1230,7 @@ static ssh_string _RSA_do_sign_hash(const unsigned char *digest,
     }
 
     ssh_string_fill(sig_blob, sig, slen);
-    memset(sig, 'd', slen);
+    explicit_bzero(sig, slen);
     SAFE_FREE(sig);
 
     return sig_blob;