X.509: Fix out-of-bounds access when parsing extensions

In the Linux kernel, the following vulnerability has been resolved:

X.509: Fix out-of-bounds access when parsing extensions

Leo reports an out-of-bounds access when parsing a certificate with

empty Basic Constraints or Key Usage extension because the first byte of

the extension is read before checking its length. Fix it.

The bug can be triggered by an unprivileged user by submitting a

specially crafted certificate to the kernel through the keyrings(7) API.

Leo has demonstrated this with a proof-of-concept program responsibly

disclosed off-list.