Fix sed s//\[newline]/ line continuations.

Fix sed s//\[newline]/ line continuations.

The problem was that readline() was returning a newline at the end of each
string, which wasn't getting stripped in the parser and thus \ wasn't at the
end of a line for -f, it was escaping a literal newline, so the continuation
logic didn't trigger. Remove some redundant null checks while we're at it,
and don't bother terminating a string we don't return (yes we leak memory
in an error path, but it's about to error_exit() anyway).