fix #K4006 - 'null obj after quiver merge' panic (NetHack 3.7)

Using apply to unlight a lit potion of oil makes it unlit, removes
it from inventory, and then re-adds it to try to force it to merge
with other potions of oil.  If it was wielded and the other potions
were quivered, the game would panic.  When merging, they get forced
into the weapon slot in preference to the quiver slot.

Unwearing it before freeinv+addinv would solve this but also leave
the hero with nothing wielded, even if it didn't merge with another
stack.  Instead, don't try to merge if the potion being unlit happens
to be worn.

hackem_changelog.txt

@@ -5,6 +5,7 @@
 
 Version 1.2.1 (unreleased)
 
+fix #K4006 - 'null obj after quiver merge' panic (NetHack 3.7)
 Gave fire bolt a little to-hit boost.
 Chaotic dragons can betray you (from SLASH'EM).
 Fix: follow-up to last commit (kicking and martial arts) (From EvilHack).

src/apply.c

@@ -1674,10 +1674,14 @@ struct obj **optr;
         /*
          * Free & add to re-merge potion.  This will average the
          * age of the potions.  Not exactly the best solution,
-         * but its easy.
+         * but its easy.  Don't do that unless obj is not worn (uwep,
+         * uswapwep, or uquiver) because if wielded and other oil is
+         * quivered a "null obj after quiver merge" panic will occur.
          */
-        freeinv(obj);
-        *optr = addinv(obj);
+        if (!obj->owornmask) {
+            freeinv(obj);
+            *optr = addinv(obj);
+        }
         return;
     } else if (Underwater) {
         There("is not enough oxygen to sustain a fire.");