Fix a double-free that would occur on an invalid cert in a CERTS cell

We would stash the certs in the handshake state before checking them
for validity... and then if they turned out to be invalid, we'd give
an error and free them.  Then, later, we'd free them again when we
tore down the connection.

Fixes bug 4343; fix on 0.2.3.6-alpha.

changes/bug4343

@@ -0,0 +1,5 @@
+  o Major bugfixes:
+    - Fix a double-free bug that would occur when we received an invalid
+      certificate in a CERT cell in the new v3 handshake. Fixes bug 4343;
+      bugfix on 0.2.3.6-alpha.
+

src/or/command.c

@@ -1020,8 +1020,6 @@ command_process_cert_cell(var_cell_t *cell, or_connection_t *conn)
       ERR("The certs we wanted were missing");
 
     /* Remember these certificates so we can check an AUTHENTICATE cell */
-    conn->handshake_state->id_cert = id_cert;
-    conn->handshake_state->auth_cert = auth_cert;
     if (! tor_tls_cert_is_valid(auth_cert, id_cert, 1))
       ERR("The authentication certificate was not valid");
     if (! tor_tls_cert_is_valid(id_cert, id_cert, 1))
@@ -1032,6 +1030,8 @@ command_process_cert_cell(var_cell_t *cell, or_connection_t *conn)
              safe_str(conn->_base.address), conn->_base.port);
     /* XXXX check more stuff? */
 
+    conn->handshake_state->id_cert = id_cert;
+    conn->handshake_state->auth_cert = auth_cert;
     id_cert = auth_cert = NULL;
   }