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

table/format.h

@@ -112,14 +112,10 @@ class Footer {
   static const uint64_t kInvalidTableMagicNumber = 0;
 
  private:
-  // Set the table_magic_number only when it was not previously
-  // initialized.  Return true on success.
-  bool set_table_magic_number(uint64_t magic_number) {
-    if (HasInitializedTableMagicNumber()) {
-      table_magic_number_ = magic_number;
-      return true;
-    }
-    return false;
+  // REQUIRES: magic number wasn't initialized.
+  void set_table_magic_number(uint64_t magic_number) {
+    assert(!HasInitializedTableMagicNumber());
+    table_magic_number_ = magic_number;
   }
 
   // return true if @table_magic_number_ is set to a value different
@@ -130,7 +126,7 @@ class Footer {
 
   BlockHandle metaindex_handle_;
   BlockHandle index_handle_;
-  uint64_t table_magic_number_;
+  uint64_t table_magic_number_ = 0;
 };
 
 // Read the footer from file