change the init parameter for FileDescriptor

Summary:
  fix a bug in improve_file_key_search, change the parameter for FileDescriptor

Test Plan:
  make all check

Reviewers: sdong

Reviewed By: sdong

Differential Revision: https://reviews.facebook.net/D19611

db/version_edit.h

@@ -99,7 +99,7 @@ struct FdWithKeyRange {
   Slice largest_key;     // slice that contain largest key
 
   FdWithKeyRange()
-      : fd(0, 0),
+      : fd(),
         smallest_key(),
         largest_key() {
   }

db/version_set_test.cc

@@ -118,7 +118,7 @@ class FindLevelFileTest {
     // add compressd_level_
     size_t num = level_files_.num_files;
     auto& file = level_files_.files[num];
-    file.fd = FileDescriptor(num + 1, 0);
+    file.fd = FileDescriptor(num + 1, 0, 0);
     file.smallest_key = Slice(mem, smallest_slice.size());
     file.largest_key = Slice(mem + smallest_slice.size(),
         largest_slice.size());