gas: don't get confused by .asci{i,z} after .struct

gas: don't get confused by .asci{i,z} after .struct

While not allowed, this certainly shouldn't result in confusing the
programmer (by skipping lines in unexpected ways): Without returning,
demand_empty_rest_of_line() (at the end of the function) will demand
the _next_ line to be empty, and without the conditional we would
ignore the next line.