generic object fix

When hallucinating, random object selection for objects was including
the new generic objects.  It was already excluding 'strange object'
by using 'rn2(NUM_OBJECTS - 1) + 1' to skip objects[0]; changing that
to be 'rn2(NUM_OBJECTS - MAXOCLASSES) + MAXOCLASSES' will skip the
first 18 objects, 'strange object' plus the 17 generic objects.

(I'm trying to convince myself that there's no off-by-1 or off-by-N
error and think I've succeeded.)

doc/fixes3-7-0.txt

@@ -1457,6 +1457,10 @@ adding command line 'nethack --usage' broke 'nethack -u name' (however,
 being hit by a big monster and getting knockback effect could send hero out
 	of a shop (or into its "free spot") while carrying unpaid goods;
 	robbery wasn't noticed until hero eventually moved to a different spot
+hallucination of objects incorrectly included generic objects as candidates
+	for what objects looked like; if an object had its dknown flag set,
+	formatting a generic object in its place was unreliable (Null pointer
+	deference prior to static analyzer fix, odd fake name "generic" after)
 
 curses: 'msg_window' option wasn't functional for curses unless the binary
 	also included tty support