tpm2: Check size of buffer before accessing it (CVE-2023-1017 & -1018)

tpm2: Check size of buffer before accessing it (CVE-2023-1017 & -1018)

Check that there are sufficient bytes in the buffer before reading the
cipherSize from it. Also, reduce the bufferSize variable by the number
of bytes that make up the cipherSize to avoid reading and writing bytes
beyond the buffer in subsequent steps that do in-place decryption.

This fixes CVE-2023-1017 & CVE-2023-1018.

Signed-off-by: Stefan Berger <stefanb@linux.ibm.com>