fix #H30 - rn2(0) from off by 1 bug in special level door creation

From a bug report, the placement
of random doors by the code that loads special levels would attempt to
evaluate rn2(0) and either get a divide by zero crash (normal build) or an
impossible warning (DEBUG enabled when compiing rnd.c, done automatically
when BETA is defined).  The problem was only noticable for random door in
a 1x1 room; none of our distributed levels specify such a thing so regular
users won't have encountered this bug.  It's a one line fix.

     Altar placement in temples also had a quirk of a similar nature.  It
wouldn't trigger rn2(0) problems but would always place the altar to left
of mid-point in rooms with even width and above the center point in ones
with even height.  Now the placement is randomized so that sometimes it'll
be to the right and/or below mid-point in such cases.

     This also simplifies a couple other instances of similar expressions
that I spotted.

doc/fixes34.4

@@ -181,6 +181,8 @@ only count successful statue creations against the monster limit in sp_lev.c
 unseen wand of striking zapped by unseen monster became known if it hit a door
 tweak knight quest messages
 guidebook grammar bits
+special level loader wasn't able to place random door in 1x1 room; could
+	trigger divide-by-0 crash for user-developed custom levels
 
 
 Platform- and/or Interface-Specific Fixes