Fix: mksobj_init could generate stack of multiple thiefstones

thiefstone_init doesn't set the thiefstone's quan (as it shouldn't), and
the value was left uninitialized in a thiefstone's path through
mksobj_init. Fix this by explicitly setting quan to 1. Move the
thiefstone check up higher so the no-stacks-of-luckstones check doesn't
need to also check thiefstones.

src/mkobj.c

@@ -1076,11 +1076,12 @@ mksobj_init(struct obj *otmp, boolean artif)
         otmp->corpsenm = 0; /* LOADSTONE hack */
         if (otmp->otyp == ROCK)
             otmp->quan = (long) rn1(6, 6);
-        else if (otmp->otyp != LUCKSTONE && otmp->otyp != THIEFSTONE
-                 && !rn2(6))
-            otmp->quan = 2L;
-        else if (otmp->otyp == THIEFSTONE)
+        else if (otmp->otyp == THIEFSTONE) {
             init_thiefstone(otmp);
+            otmp->quan = 1L;
+        }
+        else if (otmp->otyp != LUCKSTONE && !rn2(6))
+            otmp->quan = 2L;
         else
             otmp->quan = 1L;
         break;