The previous fixes for metadata_iterators didn't completely fix the problem.

The behavior of chain_prepare_for_write_() must always be the same as the
behavior of FLAC__metadata_chain_check_if_tempfile_needed(). Before this
fix, one check was missing in FLAC__metadata_chain_check_if_tempfile_needed(),
and also chain_prepare_for_write_() checked the sizes of the metadata blocks
*after* making the changes to the chain, while
FLAC__metadata_chain_check_if_tempfile_needed() does it *before* the changes.

This patch changes FLAC__metadata_chain_check_if_tempfile_needed() so that it
keeps some info (lbs_state, lbs_size) about estimated changes and then uses
it to check the block sizes.

It also simplifies FLAC__metadata_chain_check_if_tempfile_needed() a little.

Patch-from: lvqcl <lvqcl.mail@gmail.com>