pki: Fix a memory leak in ssh_pki_do_sign()

CID 1395335

Signed-off-by: Andreas Schneider <asn@cryptomilk.org>
(cherry picked from commit 8323cd791f)

src/pki.c

@@ -1903,8 +1903,10 @@ ssh_string ssh_pki_do_sign(ssh_session session,
             SSH_LOG(SSH_LOG_TRACE, "Unknown hash algorithm for type: %d",
                     sig->type);
             ssh_string_free(session_id);
+            ssh_buffer_free(buf);
             return NULL;
         }
+        ssh_buffer_free(buf);
 
 #ifdef DEBUG_CRYPTO
         ssh_print_hexa("Hash being signed", hash, hlen);