iv-gen: Ensure external sequential IVs are actually sequential

We allow gaps in IVs, but ensure that an IV is never used more than once.

src/libstrongswan/crypto/iv/iv_gen_seq.h

@@ -25,6 +25,9 @@
 /**
  * Create an IV generator that generates sequential IVs (counter).
  *
+ * The passed external IV must be larger than the one passed to any previous
+ * call.
+ *
  * @return		IV generator
  */
 iv_gen_t *iv_gen_seq_create();