be: assert that be_parse_arg() is only called before be is initialized
be: assert that be_parse_arg() is only called before be is initialized

Without the assert it is easy to run into suble bugs because
be_get_backend_param() was called before all arguments are set by
be_parse_arg().

MatzeB/libfirm,jonashaag/libfirm,libfirm/libfirm,MatzeB/libfirm,MatzeB/libfirm,libfirm/libfirm,MatzeB/libfirm,jonashaag/libfirm,jonashaag/libfirm,jonashaag/libfirm,jonashaag/libfirm,MatzeB/libfirm,libfirm/libfirm,libfirm/libfirm,MatzeB/libfirm,jonashaag/libfirm,libfirm/libfirm,jonashaag/libfirm,MatzeB/libfirm