[lib] Switch Fletcher checksum back to old ospfd version

* lib/checksum.c: (fletcher_checksum) Switch the second phase of the checksum
  back to the old ospfd logic.

  The isisd-derived version:

  a) is very hard to follow
  b) had some kind of subtle bug that caused it be wrong when c0=0 and c1=254
     (potentially fixable by doing the mods before adjusting x and y)

  Additionally:

  - explicitely cast expressions using non-internal variables to int, to ensure
    the result is signed.
  - defensively change the length argument to 'size_t', to ensure the code
    works with that argument being unsigned..

  Thanks to Joakim Tjernlund for the investigative work into this bug.

* tests/test-checksum.c: new file to exercise the checksum code.

1 files changed, 1 insertions, 1 deletions

diff --git a/lib/checksum.h b/lib/checksum.h
index d3ce9302..da1d3cba 100644
--- a/lib/checksum.h
+++ b/lib/checksum.h
@@ -1,2 +1,2 @@
 extern int in_cksum(void *, int);
-extern u_int16_t fletcher_checksum(u_char * buffer, int len, u_int16_t offset);
+extern u_int16_t fletcher_checksum(u_char *, const size_t len, const uint16_t offset);