(identify_blocks): Handle case where no block nodes
exist.  Use xmalloc not alloca for block_vector.
(reorder_blocks): New argument top_block.  Handle case where no
block nodes exist.  Always clear BLOCK_SUBBLOCKS (block) when
adding it to the block tree.
(all_blocks): Handle case where vector is zero. Store current
block at offset 0 in vector, not n_blocks.  Don't count current
block twice in n_blocks.

From-SVN: r1797