Fix error handling with getline

The function getline is used to read data from different streams, however,
the error handling is not accurate.

From the man page, getline returns -1 on failure and sets errno accordingly,
but the current implementation only checks errno to see if there was a
failure. With this approach, in case getline returns success but also sets
errno it is consider and error.

Fix the issue but also checking the return value of getline.

Forwarded: https://salsa.debian.org/apt-team/apt/-/merge_requests/265



Signed-off-by: Walter Lozano <walter.lozano@collabora.com>