Don't crash on truncated tar archives (#2364)

Don't crash on truncated tar archives (#2364)

The tar header parsing overhaul in #2127 introduced a systematic
mishandling of truncated files: the code incorrectly checks for whether
a given read operation failed, and ends up dereferencing a NULL pointer
in this case. I've gone back and double-checked how
`__archive_read_ahead` actually works (it returns NULL precisely when it
was unable to satisfy the read request) and reworked the error handling
for each call to this function in archive_read_support_format_tar.c

Resolves #2353
Resolves https://issues.oss-fuzz.com/issues/42537231