Audit: better estimation of execve record length

The audit execve record splitting code estimates the length of the message
generated.  But it forgot to include the "" that wrap each string in its
estimation.  This means that execve messages with lots of tiny (1-2 byte)
arguments could still cause records greater than 8k to be emitted.  Simply
fix the estimate.

Signed-off-by: Eric Paris <eparis@redhat.com>