read_icb_extents parsed a continuation block's allocation descriptors from
offset 0, but a continuation block begins with a 24-byte Allocation Extent
Descriptor (ECMA-167 4/14.5): 16-byte descriptor tag + prev-loc(4) +
length_of_allocation_descriptors(4); the real ADs start at offset 24. Reading
from 0 mis-parsed the AED tag as a descriptor -> one garbage extent, then an
unknown-type break, silently truncating any file whose extents spill into a
continuation block.
Few-extent files store their ADs inline and never hit this path, so it stayed
hidden; a heavily fragmented file (~1600 fragments) truncated to 113 extents +
a bogus non-unit-aligned fragment that then aborted the mux
(ExtentNotUnitAligned). Skip the 24-byte AED header, read l_ad bytes of
descriptors from offset 24. Adds a regression test with a non-zero AED tag.
Matthew Jackson
ebf30a679e