fix github issue #731 - accessing freed memory \

after charging causes a ring to explode

Reported by gebulmer:  if charging exploded a ring, the ring's memory
got freed but the stale pointer was passed to cap_spe() which accessed
it again.  Fix by setting the object pointer to Null after using up
the ring.  This was a post-3.6 bug.

Fixes #731

src/read.c

@@ -786,7 +786,7 @@ recharge(struct obj* obj, int curse_bless)
             if (is_on)
                 Ring_gone(obj);
             s = rnd(3 * abs(obj->spe)); /* amount of damage */
-            useup(obj);
+            useup(obj), obj = 0;
             losehp(Maybe_Half_Phys(s), "exploding ring", KILLED_BY_AN);
         } else {
             long mask = is_on ? (obj == uleft ? LEFT_RING : RIGHT_RING) : 0L;