server: free strduped port numbers

Although the leak is negligible, the clean heap on exit will ease
valgrind testing.

Change-Id: I3a7a9c8e8dc7557aa51d0b9caa244537e5e7007d
Signed-off-by: Tomas Vanek <vanekt@fbl.cz>
Reviewed-on: http://openocd.zylin.com/4410
Tested-by: jenkins

src/server/gdb_server.c

@@ -3572,3 +3572,9 @@ int gdb_register_commands(struct command_context *cmd_ctx)
 	gdb_port_next = strdup("3333");
 	return register_commands(cmd_ctx, NULL, gdb_command_handlers);
 }
+
+void gdb_service_free(void)
+{
+	free(gdb_port);
+	free(gdb_port_next);
+}