lost ball and chain (trunk only)

Saving the game while punished, not carrying the attached ball,
and while swallowed by a purple worm resulted in losing the
ball and chain.

Since the required information was not being written to the
save file at all, I couldn't come up with a clean way to do this
for the branch, and preserve save file format. I could think
of lots of kludgy ways to do it (insert ball and chain into
the hero's inventory prior to saving, and remove it on restore, etc.)

src/save.c

@@ -347,6 +347,13 @@ register int fd, mode;
 	if (!release_data(mode))
 		put_gold_back(&invent, &u.ugold);
 #endif
+	if (BALL_IN_MON) {
+		/* prevent loss of ball & chain when swallowed */
+		uball->nobj = uchain;
+		uchain->nobj = (struct obj *)0;
+		saveobjchn(fd, uball, mode);
+	}
+
 	saveobjchn(fd, migrating_objs, mode);
 	savemonchn(fd, migrating_mons, mode);
 	if (release_data(mode)) {