handle junk bytes after tree

[zacharyburnett]

Description

fixes asdf-format/asdf-standard#441

• junk before blocks (after tree) raises OSError: Invalid block magic
  

  asdf/asdf/_block/reader.py

  Lines 249 to 257 in 867c280

  	try:
  	# load first and last blocks to check if the index looks correct
  	for index in (0, -1):
  	fd.seek(block_index[index])
  	buff = fd.read(magic_len)
  	if buff != constants.BLOCK_MAGIC:
  	msg = "Invalid block magic"
  	raise OSError(msg)
  	blocks[index].load()

• junk after blocks (after tree) raises asdf.exceptions.AsdfBlockIndexWarning: Invalid block index contents for block 0, falling back to serial reading: Invalid block magic
  

  asdf/asdf/_block/reader.py

  Lines 258 to 262 in 867c280

  	except (OSError, ValueError) as e:
  	msg = f"Invalid block index contents for block {index}, falling back to serial reading: {e!s}"
  	warnings.warn(msg, AsdfBlockIndexWarning)
  	fd.seek(starting_offset)
  	return _read_blocks_serially(fd, memmap, lazy_load, after_magic)

Checklist:

• pre-commit checks ran successfully
• tests ran successfully
• for a public change, a changelog entry was added
• for a public change, documentation was updated
• for any new features, unit tests were added

[zacharyburnett]

we've decided to move these reference files to asdf-standard, and cover the following cases:

• 0 bytes
• 5 bytes