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

doc/fixes3-7-0.txt

@@ -1146,6 +1146,7 @@ add '#tip' for containers to context-sensitive invent handling
 sequencing confusion: picking an item when viewing inventory and picking an
 	action to do with it caused the inventory command to use time, then
 	on next turn the action was performed without taking any time
+program would access freed memory if charging caused a ring to explode
 
 curses: 'msg_window' option wasn't functional for curses unless the binary
 	also included tty support

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;