functions passed a chain explicitly NO_NONNULLS

Some functions are passed an obj or monst chain,
and  the callers typically don't check them
against 0, so mark them explicitly as NO_NONNULLS

(NO_NONNULLS expands to nothing, but it flags that
some null arg analysis has been done)

src/questpgr.c

@@ -14,7 +14,8 @@
 #endif
 
 static const char *intermed(void);
-static struct obj *find_qarti(struct obj *);
+/* sometimes find_qarti(gi.invent), and gi.invent can be null */
+static struct obj *find_qarti(struct obj *) NO_NONNULLS;
 static const char *neminame(void);
 static const char *guardname(void);
 static const char *homebase(void);