gas: improve interaction between read_a_source_file() and s_linefile()

gas: improve interaction between read_a_source_file() and s_linefile()

read_a_source_file() would bump line numbers only when seeing a newline,
whereas is_end_of_line[] indicates further end-of-line characters, in
particular the nul character. s_linefile() attempts to compensate for
the bump, but was too aggressive with this so far: It should only adjust
when a newline ends the line. To facilitate such a check, the check for
nothing else on the line needs to move ahead, which luckily is easily
possible: The relevant two conditions match, and the function can
simply return from the body of that earlier instance of the conditional.

The more strict treatment in s_linefile() then requires an adjustment
to buffer_and_nest()'s invocation of the function: The line terminator
now needs to be a newline, not nul.