Use NO_MATERIAL instead of a magic number

include/objclass.h

@@ -10,6 +10,7 @@
    (liquid potion inside glass bottle, metal arrowhead on wooden shaft)
    and object definitions only specify one type on a best-fit basis */
 enum obj_material_types {
+    NO_MATERIAL =  0,
     LIQUID      =  1, /* currently only for venom */
     WAX         =  2,
     VEGGY       =  3, /* foodstuffs */

src/uhitm.c

@@ -1732,7 +1732,7 @@ hmon_hitmon(
     hmd.silverobj = FALSE;
     hmd.lightobj = FALSE;
     hmd.material = obj ? objects[obj->otyp].oc_material
-                       : 0; /* 0 == NO_MATERIAL */
+                       : NO_MATERIAL;
     hmd.jousting = 0;
     hmd.hittxt = FALSE;
     hmd.get_dmg_bonus = TRUE;