Old oop-read.h bugfixes from Ian Jackson

Some years ago I contributed a feature for reading lines and records
to liboop: oop-read.h and read.c.  Since it took a while for that
feature to make it into distributed versions, for a long time I've
been using my own copy of the source file.  It seems that I fixed a
couple of bugs in my copy which are still in the Debian package.  I
can't find any record of me having told anyone about them and now I
find that 1.0-6 still has the bugs.

There are two fixes:
 * Initialise "rd->discard" properly
 * Avoid rd->neednotcheck becoming negative