libsparse: use strcmp and validate last_used pointer

This patch is used to fix two Bugs in backed_block.c

First, fix wrong comparing string way: we should use strcmp rather than
just compare their address. Second, fix using illegal memory risk in
bbl->last_used pointer. When entering queue_bb function,
bbl->last_used = new_bb, but in the following code if
queue_bb(xx, bb, new_bb) return ok, the space of new_bb is released. So
next time, if you use bbl->last_used pointer, may cause segment fault !

Change-Id: I6abb505f9b903b697448639fc64fb7518df5cca1