[release-branch.go1.25] encoding/pem: properly decode strange PEM data
When the passed byte slice has leading garbage, properly handle ignoring
it and continuing to parse the slice until we find a valid block (or
nothing).
Fixes #75952
Change-Id: I07e937d9c754fd71b028b99450b48f57b4464457
Reviewed-on: https://go-review.googlesource.com/c/go/+/712140
Reviewed-by: Damien Neil <dneil@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
(cherry picked from commit
09830901714d8b3a2cc5fb33e87a81886b21ea24)
Reviewed-on: https://go-review.googlesource.com/c/go/+/712640
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>