gas: fix rs_fill_nop listing

In commit a0094f1a70e1 ("gas: make .nops output visible in listing") I
was wrongly assuming fr_fix would be zero for rs_fill_nop, when that's
only a side effect of listing_newline() inserting dummy frags, but only
when file/line did actually change from the previous invocation. This is
in particular not going to be true when the .nops directive isn't the
first statement on a line.