ipsec: validate number of input sas

Type: fix

There is a statically allocated array for inbound SAs which can hold
4 IDs. The input parameter containing the IDs of th inbound SAs is a
vector and Its possible to pass a vector with more than 4 elements
and write the memory past the end of the array. Fail if more than 4
SAs are passed in the vector.

Change-Id: I0c9d321c902d6366b8aff816d04e343dcbd110eb
Signed-off-by: Matthew Smith <mgsmith@netgate.com>