Only get the manifest file size if there is no error

Summary:
I came across this while working on column families. CorruptionTest::RecoverWriteError threw a SIGSEG because the descriptor_log_->file() was nullptr. I'm not sure why it doesn't happen in master, but better safe than sorry.

@kailiu, can we get this in release, too?

Test Plan: make check

Reviewers: kailiu, dhruba, haobo

Reviewed By: haobo

CC: leveldb, kailiu

Differential Revision: https://reviews.facebook.net/D15513

db/version_set.cc

@@ -1547,8 +1547,10 @@ Status VersionSet::LogAndApply(VersionEdit* edit, port::Mutex* mu,
       }
     }
 
-    // find offset in manifest file where this version is stored.
-    new_manifest_file_size = descriptor_log_->file()->GetFileSize();
+    if (s.ok()) {
+      // find offset in manifest file where this version is stored.
+      new_manifest_file_size = descriptor_log_->file()->GetFileSize();
+    }
 
     LogFlush(options_->info_log);
     mu->Lock();