backup never runs when max_execution_time = 0

ini_get('max_execution_time') returns 0 on my development system. (Don't ask me why.)

The issue is that the logic of _backup_migrate_check_timeout() is such, that no backups run if ini_get('max_execution_time') < variable_get('backup_migrate_timeout_buffer', 5).

So I made the code a little more robust.

I included a patch without whitespace changes, but that messes the indentation up a little. So I included the same patch with whitespace changes as well, for applying. (Is correct, but more confusing to read.)