crypto/x509: avoid panic when parsing partial PKCS#1 private keys

crypto/x509: avoid panic when parsing partial PKCS#1 private keys

These keys are off-spec, but have historically been accepted by
ParsePKCS1PrivateKey.

Thanks to Philippe Antoine (Catena cyber) for reporting this issue.

Fixes #71216
Fixes CVE-2025-22865

Change-Id: I6a6a46564156fa32e29e8d6acbec3fbac47c7352
Reviewed-on: https://go-internal-review.googlesource.com/c/go/+/1820
Reviewed-by: Tatiana Bradley <tatianabradley@google.com>
Reviewed-by: Damien Neil <dneil@google.com>
Commit-Queue: Roland Shoemaker <bracewell@google.com>
Reviewed-on: https://go-review.googlesource.com/c/go/+/643098
Auto-Submit: Michael Knyszek <mknyszek@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Pratt <mpratt@google.com>