encoding/pem: malformed line endings can cause panics

[rolandshoemaker]

Fixed by https://go.dev/cl/714200.

[rolandshoemaker]

@gopherbot please open backports, this is a regression in PEM decoding.

[gopherbot]

Backport issue(s) opened: #76028 (for 1.24), #76029 (for 1.25).

Remember to create the cherry-pick CL(s) as soon as the patch is submitted to master, according to https://go.dev/wiki/MinorReleases.

[dolmen]

The prefix of the title of this issue should be encoding/pem instead of pem/encoding.