additonal oextra care

Be deliberately careful with copies taken of
oextra pointers and clear the pointer if it
truly is a redundant copy that will become
invalid if/when the original holder is deallocated.

src/trap.c

@@ -2748,7 +2748,8 @@ domagictrap()
 		   {	struct obj pseudo;
 			long save_conf = HConfusion;
 
-			pseudo = zeroobj;   /* neither cursed nor blessed */
+			pseudo = zeroobj;   /* neither cursed nor blessed,
+						and zero out oextra */
 			pseudo.otyp = SCR_REMOVE_CURSE;
 			HConfusion = 0L;
 			(void) seffects(&pseudo);