Fix UBSan build break caused by variable not initialized

Summary: UBSan is unhappy because `cfd` is not initialized. This breaks UBSan build which in turn breaks MyRocks continuous integration with RocksDB which in turns makes me unhappy :-) Fix this.

Test Plan:
- `[p]arc diff --preview` + Sandcastle.
- Verify that `COMPILE_WITH_UBSAN=1 OPT=-g make J=1 ubsan_check` gets past the break.

Reviewers: andrewkr, hermanlee4, sdong

Reviewed By: sdong

Subscribers: andrewkr, dhruba

Differential Revision: https://reviews.facebook.net/D60117

db/repair.cc

@@ -449,7 +449,7 @@ class Repairer {
             AddColumnFamily(props->column_family_name, t->column_family_id);
       }
     }
-    ColumnFamilyData* cfd;
+    ColumnFamilyData* cfd = nullptr;
     if (status.ok()) {
       cfd = vset_.GetColumnFamilySet()->GetColumnFamily(t->column_family_id);
       if (cfd->GetName() != props->column_family_name) {