btrfs-progs: avoid double-free in __btrfs_map_block

__btrfs_map_block() can possibly do the goto again: loop after
having allocated & freed the "multi" pointer.  There are then
a couple error conditions where it will attempt to again kfree
the now non-NULL multi pointer.  So before retrying, reset
multi to NULL after we free it.

Signed-off-by: Eric Sandeen <sandeen@redhat.com>

volumes.c

@@ -1226,6 +1226,7 @@ again:
 	if (multi_ret && stripes_allocated < stripes_required) {
 		stripes_allocated = stripes_required;
 		kfree(multi);
+		multi = NULL;
 		goto again;
 	}
 	stripe_nr = offset;