apk: handle edge case when parsing .apk files

apk: handle edge case when parsing .apk files

This was a regression introduced in the recent alignment changes and led
to failures when reading (i.e. 'mkndx') certain packages like follows:

ERROR: python3-botocore-1.31.7-r1.apk: unexpected end of file

It affected packages with a header size greater than the read buffer
size of 128KB but less than 160KB (128KB + (128KB / 4)).

In those cases, we'd attempt a 0 byte read, leading to APKE_EOF.

Based on some tests of files across multiple archs and feeds, it seems
the only packages meeting those criteria were python3-botocore and
golang-github-jedisct1-dnscrypt-proxy2-dev.

Fixes: 64ec08eee1 ("apk: backport upstream fixes for unaligned access")
Signed-off-by: Matt Merhar <mattmerhar@protonmail.com>
Link: https://github.com/openwrt/openwrt/pull/21992
Signed-off-by: Robert Marko <robimarko@gmail.com>