Clarify crc32 variable names (#2367)

Clarify crc32 variable names (#2367)

No functional change, just a tiny style improvement.

Use `crc32_computed` to refer to the crc32 that the reader has computed
and `crc32_read` to refer to the value that we read from the archive.
That hopefully makes this code a tiny bit easier to follow. (It confused
me recently when I was double-checking something in this area, so I
thought an improvement here might help others.)