Fire sources can ignite candles, lamps, and potions of oil

... on the floor, in monster inventory, and in hero's inventory.

Items in your inventory being ignited produce a message even if you're
blind - you can see the lit-state by viewing inventory anyway, so just
give player the message.

(via xNetHack)

src/mhitm.c

@@ -1042,6 +1042,7 @@ int dieroll;
         }
         /* only potions damage resistant players in destroy_item */
         tmp += destroy_mitem(mdef, POTION_CLASS, AD_FIRE);
+        ignite_items(mdef->minvent);
         break;
     case AD_COLD:
         if (cancelled) {