Fix two bugs in talbe format

Summary:
Previous code had two bugs:

* didn't initialize the table_magic_number_ explicitly -- as a
  result a random junk number is stored for table_magic_number_, making
  HasInitializedMagicNumber() always return true.
* if condition is inconrrect in set_table_magic_number(), and the return value is not checked.
  I replace if-else by a stronger requirement enforced by assert().

Test Plan:
Previous sst_dump failed to work.
After the fix, things back to normal.

Reviewers: yhchiang

CC: haobo, sdong, igor, dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D17055