U295 - bones file creation cursing secondary weapon caused panic

As reported, if you're twoweaponing and die, your secondary weapon may
become cursed and drop.  But, the bones file code is dropping everything
and tries to drop it again, causing a panic.  drop_upon_death just clears
things out, so follow suit for uswapwep.

src/bones.c

@@ -124,6 +124,7 @@ struct obj *cont;
 {
 	struct obj *otmp;
 
+	uswapwep = 0; /* ensure curse() won't cause swapwep to drop twice */
 	while ((otmp = invent) != 0) {
 		obj_extract_self(otmp);
 		obj_no_longer_held(otmp);