Fix: potential use-after-free when monster threw lit oil

Specifically at melee range, and when the monster was carrying more oil
that could ignite and explode. The code previously assumed that if there
was more than 1 oil in the stack to throw that it would definitely still
be around after the explosion to read the memory contents of, which was
not true.

Another ASan catch.

src/muse.c

@@ -1716,7 +1716,7 @@ use_offensive(struct monst* mtmp)
          * are not objects.  Also set dknown in mthrowu.c.
          */
         boolean isoil = (otmp->otyp == POT_OIL);
-        int origquan = otmp->quan;
+        struct obj *minvptr;
         if (cansee(mtmp->mx, mtmp->my)) {
             otmp->dknown = 1;
             pline("%s hurls %s!", Monnam(mtmp), singular(otmp, doname));
@@ -1730,9 +1730,18 @@ use_offensive(struct monst* mtmp)
         m_throw(mtmp, mtmp->mx, mtmp->my, sgn(mtmp->mux - mtmp->mx),
                 sgn(mtmp->muy - mtmp->my),
                 distmin(mtmp->mx, mtmp->my, mtmp->mux, mtmp->muy), otmp);
-        if (isoil && origquan > 1 && otmp->lamplit) {
-            /* origquan > 1: otmp is still valid */
-            end_burn(otmp, TRUE);
+        if (isoil) {
+            /* Possible situation: monster lights and throws 1 of a stack of oil
+             * point blank -> it explodes -> monster is caught in explosion ->
+             * monster's remaining oil ignites and explodes -> otmp is no longer
+             * valid. So we need to check whether otmp is still in monster's
+             * inventory or not. */
+            for (minvptr = mtmp->minvent; minvptr; minvptr = minvptr->nobj) {
+                if (minvptr == otmp)
+                    break;
+            }
+            if (minvptr == otmp && otmp->lamplit)
+                end_burn(otmp, TRUE);
         }
         return 2;
     }