Reset size_ to 0 in PinnableSlice::Reset (#4962)

Summary:
It would avoid bugs if the reused PinnableSlice is not actually reassigned and yet the programmer makes conclusions based on the size of the Slice.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/4962

Differential Revision: D14012710

Pulled By: maysamyabandeh

fbshipit-source-id: 23f4e173386b5461fd5650f44cde470805f4e816

include/rocksdb/slice.h

@@ -202,6 +202,7 @@ class PinnableSlice : public Slice, public Cleanable {
   void Reset() {
     Cleanable::Reset();
     pinned_ = false;
+    size_ = 0;
   }
 
   inline std::string* GetSelf() { return buf_; }