Block parsing allows a few (<4) bytes of junk data after the last block

[braingram]

This is likely a bug but the following test relies on this behavior:
https://github.com/asdf-format/asdf/blob/3d46a99c69a06dcb806606190cc7815e15483a3c/asdf/_tests/test_array_blocks.py#L445C5-L465

Reading files with a streamed block also depends on this behavior as fd.fast_forward(-1) results in a seek to the last byte in the file (not the end of the file) so reading the input file after the streamed block is encountered results in reading the last byte in the stream.

[braingram]

This is partially addressed in #1537 which introduced a warning:

asdf/asdf/_block/reader.py

Lines 142 to 145 in 9680015

	# if these are non-0 bytes issue a warning that the file
	# is likely corrupt
	msg = f"Read invalid bytes {buff!r} after blocks, your file might be corrupt"
	warnings.warn(msg, AsdfWarning)

However a more thorough set of tests are needed.