feh_draw_info used to unconditionally swallow the last character of a line,
assuming that it must be a newline. However, the very last line of a program's
output may not end in a newline, in which case its last output character was
omitted.

This prevents a (highly unlikely) case of an attacker knowing feh's PID and
the user's URL rewriting user files by means of a TOCTTOU attack.

It is still possible to _create_ arbitrary files via dangling symlinks. That
will be fixed once I switch from wget to libcurl.

Signed-off-by: Daniel Friesel <derf@finalrewind.org>