dragon armor fix

Fix a segfault when polymorphed into a dragon and using ^X.

One inconsistency I've spotted that I hadn't noticed earlier:  if
you wear red dragon scales/mail you obtain infravision ability, but
if polymorph into a red dragon, you don't.

include/youprop.h

@@ -67,7 +67,7 @@
 #define HSick_resistance u.uprops[SICK_RES].intrinsic
 #define ESick_resistance u.uprops[SICK_RES].extrinsic
 #define Sick_resistance (HSick_resistance || ESick_resistance \
-                         || defends(AD_DISE, uwep))
+                         || defended(&g.youmonst, AD_DISE))
 
 /* Intrinsics only */
 #define Invulnerable u.uprops[INVULNERABLE].intrinsic /* [Tom] */

src/artifact.c

@@ -396,6 +396,8 @@ defends(int adtyp, struct obj *otmp)
 {
     struct artifact *weap;
 
+    if (!otmp)
+        return FALSE;
     if ((weap = get_artifact(otmp)) != 0)
         return (boolean) (weap->defn.adtyp == adtyp);
     if (Is_dragon_armor(otmp)) {