fix #K4006 - 'null obj after quiver merge' panic

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.

3.6.x was subject to this too and the fix is small+isolated but the
situation is so uncommon that I haven't bothered backporting it.

Applying a lump of royal jelly and then not picking anything to rub
it on had a similar problem.  It also panicked if the applied lump
was wielded and other lumps were quivered.  The fix is different
because the stack it gets split from during apply is known.  This one
doesn't impact 3.6.x; applying jelly to eggs wasn't implemented yet.

src/nhlua.c

@@ -1583,7 +1583,9 @@ nhl_gamestate(lua_State *L)
             wornmask = otmp->owornmask;
             otmp->owornmask = 0L;
             extract_nobj(otmp, &gmst_invent);
+            otmp->nomerge = 1;
             addinv(otmp);
+            otmp->nomerge = 0;
             if (wornmask)
                 setworn(otmp, wornmask);
         }