TPM: get_event_name stack corruption

get_event_name uses sprintf to fill a buffer declared on the stack.  It fills
the buffer 2 bytes at a time.  What the code doesn't take into account is that
sprintf(buf, "%02x", data) actually writes 3 bytes.  2 bytes for the data and
then it nul terminates the string.  Since we declare buf to be 40 characters
long and then we write 40 bytes of data into buf sprintf is going to write 41
characters.  The fix is to leave room in buf for the nul terminator.

Signed-off-by: Eric Paris <eparis@redhat.com>
Signed-off-by: James Morris <jmorris@namei.org>