nts: check for negative length in NNA_DecryptAuthEF()

As other functions that accept a signed length, make sure it is sane in
NNA_DecryptAuthEF() too.

test/unit/nts_ntp_auth.c

@@ -82,6 +82,10 @@ test_unit(void)
     TEST_CHECK(r);
     TEST_CHECK(info.length - packet_length >= min_ef_length);
 
+    r = NNA_DecryptAuthEF(&packet, &info, siv, packet_length, plaintext2,
+                          -1, &plaintext2_length);
+    TEST_CHECK(!r);
+
     r = NNA_DecryptAuthEF(&packet, &info, siv, packet_length, plaintext2,
                           sizeof (plaintext2), &plaintext2_length);
     TEST_CHECK(r);