Fix: Harbinger was not protecting worn armor from acid damage.

Just like Dichotomy needed a special case in burnarmor(), Harbinger
needed the same treatment in erode_armor().

Author: k21971

doc/evilhack-changelog.md

@@ -4002,4 +4002,5 @@ The following changes to date are:
 - Some more Caveperson role improvements
 - Adjust the artifact Keolewa shock damage so it's better than a club
   with the lightning object property
+- Fix: Harbinger was not protecting worn armor from acid damage
 

src/uhitm.c

@@ -40,6 +40,18 @@ struct monst *mdef;
 int hurt;
 {
     struct obj *target;
+    boolean youdefend;
+
+    if (!mdef)
+        return;
+
+    youdefend = (mdef == &youmonst);
+
+    /* wielding Harbinger protects worn armor */
+    if (youdefend ? wielding_artifact(ART_HARBINGER)
+                  : (MON_WEP(mdef)
+                     && MON_WEP(mdef)->oartifact == ART_HARBINGER))
+        return;
 
     /* What the following code does: it keeps looping until it
      * finds a target for the rust monster.