tipc: Prevent access of non-existent field in short message header

This patch eliminates a case where TIPC's link code could try reading
a field that is not present in a short message header.  (The random
value obtained was not being used, but the read operation could result
in an invalid memory access exception in extremely rare circumstances.)

Signed-off-by: Allan Stephens <allan.stephens@windriver.com>
Signed-off-by: David S. Miller <davem@davemloft.net>