iv-gen: Fail getting shorter IV values than the used counter size

While no algorithm actually uses such short IVs, we add a check here to ensure
we won't return just the lower bits of the counter.

src/libstrongswan/crypto/iv/iv_gen_seq.c

@@ -53,6 +53,10 @@ METHOD(iv_gen_t, get_iv, bool,
 	{
 		return FALSE;
 	}
+	if (size < sizeof(u_int64_t))
+	{
+		return FALSE;
+	}
 	if (this->prev != SEQ_IV_INIT_STATE && seq <= this->prev)
 	{
 		return FALSE;