Prevent adjusting items to the dash inventory letter

The #adjust command allowed, due to compactified buf, putting
items into the '-' inventory letter, which is usually reserved
for "empty hands", zeroobj. This caused problems down the line
when user was allowed to pick that letter for dropping.

src/invent.c

@@ -3622,7 +3622,7 @@ doorganize() /* inventory organizer by Del Lamb */
             pline1(Never_mind);
             return 0;
         }
-        if ((letter(let) && let != '@') || index(buf, let))
+        if ((letter(let) && let != '@') || index(buf, let) && let != '-')
             break; /* got one */
         if (trycnt == 5)
             goto noadjust;